Motorhome and Caravan Battery Life

Motorhome and Caravan Battery Life – how to select and manage motorhome and caravan batteries

How to get the most out of your expensive deep cycle batteries.

No question, good quality deep cycle batteries are very expensive – and I’m guessing that they are not going to get any cheaper any time soon. So how can we make sure we get the very best life out of these necessary lumps of lead and acid?

There are two factors that affect battery life above all others –

  1. The type of battery – its physical and chemical composition.
  2. How you treat the battery – how deep and how often you discharge it and how well you charge it.

Addressing the first point is quite simple – you pretty much get what you pay for in deep cycle batteries. The three most common types used in caravans and motorhomes are :

  • Flooded lead-acid – these are often just called deep cycle batteries. You need to keep these topped up with demineralised water and they must be located in a ventilated area (not inside the vehicle). These are the cheapest of the three types.
  • Gel Batteries – these batteries use a high viscosity electrolyte and are normally sealed, but can release hydrogen under some circumstances. These are normally more expensive than flooded lead-acid.
  • AGM or absorbent glass mat – these batteries have a glass mat between the lead plates that keeps the electrolyte in contact with the plates and helps prevent warping of the plates.  AGM batteries are (at least on the surface) the most expensive of the three options.

So why choose Gel or AGM?

Before I answer that, I need to explain “Depth of Discharge”. A battery is simply a device for storing energy – electrical energy. There is no magic; they are storage container (just as a bucket is a storage container for water). Depth of Discharge is how much of the stored electricity we take out.

If we have a 100aH battery (that is 100 amp hours) and we use one third of its stored energy, we can say we have discharged it to 66% – or a depth of discharge of 66%.

Lead acid batteries hate being discharged! (They hate being left in a discharged state even more!) If you want your batteries to last forever (metaphorically speaking) – don’t ever discharge them. Of course a battery that never gets discharged is about as much use as a car that never gets driven – and most things last a lot longer if you don’t use them.

I always think of discharging my batteries as being like inflicting pain on them – the deeper the discharge, the greater the pain and the shorter the life of the battery.

GEL batteries are less vulnerable to the “pain of discharge” than Flooded Lead-Acid. They have a “higher pain threshold” if you like, and can give up more of their stored energy before feeling that pain.

So now that we know about battery pain – we can understand that selecting the right battery is the first step in getting long and trouble free service from our motorhome and caravan batteries.

Selecting the correct number and capacity of batteries is the next step.

Let’s look at a simple example:

If we have a single 100aH battery and we take 50aH from it every night we are discharging it to a DOD (depth of Discharge) of 50% every day.

Now consider if we had two of these batteries. Two times 100 Ah equals 200aH (this is exactly the same as having a single 200aH battery). If we take 50 aH out of this set each night we are only taking them to a DOD of 75% and in so doing inflict far less pain on the pair of batteries. They will probably last perhaps 3 times longer than the single battery setup.

The Real World

Now this is great in theory – all this DOD and AH stuff – but what about the real world? How can we know how deep we are discharging our batteries? Can we use the battery voltage to tell us this stuff?


If I repeat one phrase more than any other when talking to fellow travellers about their electrics, it would have to be:

Battery voltage is NOT a good indicator of the state of charge of your batteries!

To put it plainly using a voltmeter to determine battery state is like trying to use a temperature gauge to determine how much fuel you have in your car.

You can read a full explanation of why this is in this article.

Unless the batteries have been completely disconnected for at least 12 – 24 hours the battery voltage will tell you nothing useful (the only exception to this it is if the voltage is very low or very high).

So if the trusty voltmeter is useless, how can we really know what is left in the batteries and just how much pain we are inflicting each day?

The answer comes in the form of a real battery monitor. These devices use a shunt (a current sensor) to measure how much charge goes into the battery and how much power you take out. So long as it knows the total battery capacity, it is fairly simple to determine how much power remains at any point in time.

Unfortunately the term “Battery Monitor” is also used to describe the $20 red-light/green-light devices that simply use voltage to try to make some guess about battery state. These devices are worth slightly less than nothing and should be avoided.

Real battery monitors use shunts (current sensors with big brass blocks) and (unfortunately) cost a lot more ($200 – $450 at the time of writing).

The battery monitor that we use is called a “Xantrex battery monitor” and operates just like the fuel gauge in your car (except that it is digital). When it says 100% I am very happy that my batteries are full. When it says 40% – I can almost hear them screaming in pain.

I cannot recommend these devices enough; they are the best thing to happen to caravan and motorhome electrics since the invention of the solar panel.

Xantrex now make two varieties of these devices – a lite version and a pro version. The lite version is about $100 less expensive and should do everything that most people will need it to. It is absolutely critical that it is installed and setup correctly. If it is incorrectly installed or configured it will feed you false information.

The strength of the Australian dollar has made purchasing these devices from the USA quite attractive and a quick search of eBay reveals a number of brand new battery monitors available from the US for around A$200 plus postage.


When I purchased my new batteries for our motorhome, I looked very closely at the specification provided by each battery manufacturer. I have calculated that given my current (full time) use of our motorhome, I should get 7 to 10 years of service from my batteries. With the proper care and treatment I see no reason why everyone should not expect the same from their battery system.

How old are your batteries? What type do you use and how do you manage them – let us all know. Leave a comment in the “Leave a Reply” box below.

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25 Responses to “Motorhome and Caravan Battery Life”

  1. Ron Slobe Says:

    Hi Gavin
    Thanks for the very informative articles, the best Ihave ever come across, especially in reference to electrical aspects of the RV.
    Unfortunetly the so called experts in the field don’t always give you the right information and definetly don’t explain how systems work in laymans terms.
    As mentioned before by your self if you are not told how your system works or aren’t given a basic diagram, how can a happy traveller use or monitor a system that has been installed.
    I have scolled through a lot of your technical advice and replies and have been able to get a lot of important information but was not able to get some of my queries answered ( but maybe I have missed it).
    We have a 5th wheeler and had (professionally installed and professionally paid for) 3x 100AH AGM Ritar batteries installed. As battery chargers we have 2x 120w solar panels, a C-tek 25amp charger( on 240 volts) and a 12v 35a charger for charging the 5th wheeler batteries while travelling.
    My questions are:
    What terminals do you connect the charger to when you have 3 batteries in parallel?
    Do all the 3 seperate charges Solar,C-tek and car charger get connected in the same way?
    Is there any problem with the C-tek charging the batteries at the same time as the solar( ie if you are in a caravan park) and the same if you are travelling when you would be using the car as charger and the solar( if its sunny)

    At present I am trying to sort out a problem with the batteries going completely flat after about 4 weeks of storage in our shed, (with no 240v power connected, no solar available and no car connected) and nothing in the 5th wheeler turned on and power being drawn.
    I have a Victron battery monitor installed which actually showed no amps being drawn.
    Thanks again for your help.
    Ron (Truron travellers)

  2. Hobo Says:

    Hi Ron,

    Thanks for the comments. It is always nice to get positive feedback on the site.

    Batteries connected in parallel are from an electrical point of view essentially one large battery. As all the +iv are connected and all the -iv terminals are connected and charging source (and load) can be connected to any of the battery terminals.

    Chargers – Yes, all charge sources are connected to the batteries in the same way. They will function fine like this (there will be a few strange things that happen due to voltage sensing – but it wont be a problem). There is no problem with more than one charge source being active at any time (we do this all the time – right now we have a wind turbine and the solar operating. When we drive we have the alternator AND the solar).

    Re batteries going flat. The first thing I would do is check that the battery monitor is correctly installed. Also keep in mind that even the tiniest discharge current (just a few milliamps) will flatten a battery given enough time (it is like a bucket with a tiny pin hole – given enough time the bucket will become empty). I would be using a clip-on DC ammeter to look for tiny loads.

    I hope this helps



  3. Ron Slobe Says:

    Thanks Gavin
    I will get the clip on ammeter to check where the discharge is coming from.
    I will also check my amautere instalation of the victron monitor, I didd follow the installations instruction complete with a shunt supplied. I connected it to the end battery negative terminal which also has some other earth connections, like the car charger.
    The solar and car charger are connected to the positive of the end battery and negative of the other end and the 240V C-tek is connected to the positive and negative of the centre battery.

    Anyway I will keep checking and see if I can find the leakage and keep you posted.

    If the 240V C-tek was left on 24/7 while in storage would that damage or reduce the life of the batteries.

    Another area I might check for possible drain is the original American Charger combined inverter 240 to 12 and fuse box load centre at the back of the 5th wheeler that had burned out and not working.
    The Auto electrician left the whole old unit ( because it had the whole 240v load centre/fuse box in) and only replaced it with a C-tek charger because all the lights all run on 12v anyway. So while the unit is connected to 240 it would automatically keep the batteries charged with no need to install another invertor.

    Thanks again Ron

  4. Hobo Says:

    When correctly installed, a shunt for a battery monitor will have one end connected to a battery terminal – that battery terminal MUST have no other wires on it – no exceptions. Any other arrangement is wrong and will give false readings. Think of a shunt as being like a “flow meter” measuring all current flowing into and out of the battery – – it simply can not measure any current that by-passes it.

    C-tec chargers are designed to be safely left attached all the time. This is far better for the batteries than letting them discharge. Even once you find and fix the leak, I would be inclined to leave the charger attached and on.

  5. Faith Gonzales Says:

    my husband and I have brought a caravan and want to be able to set up power so as to b able to go free camping and run our gear with as little fuss as possible, what is the best setup for running 240v fridge, laptop,aircon, lights etc. A generator is ok but u need a lot of fuel so we want to go solar with a inverters etc.could u please help with advises. Faith. or my husband Carlos on 0408011871 mobile.

  6. Hobo Says:

    Hi there,
    Solar is a great solution for the right situation. It has a limited capacity however. If your plan is to run AirCon and a 240v fridge then a generator is the only practical option. If you plan to travel for some time (years) then a medium sized solar system would probably save fuel costs, but a generator would still be required to run the aircon system. While solar is now many times cheaper that it was 5 years ago, it is still a reasonable investment – if the plan is to travel for a short time, or do short trips then solar would not be an economical option (although it depends on how much you like listening to generators when you are in a quiet peaceful camping spot).
    I hope this helps


  7. Kevin Morrow Says:

    Hi Guys,
    Been following you guys for a year now and what a great site.
    Just stripped out ex coach (NZ) and are wanting to put in 4 x 130amp – 12v agm and are wanting
    to end up with 24v, Are installing 2 x 195w solar panels and also have 3kva gen set,
    Any help would be much appreciated.
    Cheers Kevin

  8. Hobo Says:

    Hi Kevin,
    I’d be happy to provide help. With 4 x 12v batteries you will need to connect them in series-parallel. There are number of articles on the site relating to installing solar that you might find useful and if you have any spasific questions, feel free to ask.


  9. Laurie Says:

    I’ve just read a number of your articles about motorhome batteries. I want to ask about “managing the battery”:

    (Unfortunately) our motorhome spends most of its life parked in the driveway – hopefully this will change soon!

    We bought the new battery just before Easter. When we returned from our trip away, we left the 240V Off , and the 12V and fridge On – it continued operating for three days, and, although not operating, the fridge stayed under 8 degrees into the 4th day. However, it took a long time (a couple of days) after we put it back on the 240V to start the fridge again.

    The battery is new, but the fridge is old (2001) –

    How should we “manage” the battery?

    Should we leave the 240V switched on all the time? If so, should the internal 12V switch be On or Off?
    Should we charge up the battery and then switch both the 240 and the 12 volt Off?
    Should we charge up the battery, and let it Discharge for a couple of days?
    Should we charge up the battery, and then let it discharge Fully?

    Should we leave the 12V fridge On all the time?

    We have just had to buy a new battery (not cheap), and want it to be reliable and long-lastin

    I don’t know that you’ve addressed this issue elsewhere – if so, could you point me in that direction.

    Thanks in anticipation,


  10. Hobo Says:

    The best thing you can do for a lead acid battery is leave it on trickle charge all the time. Assuming the charger is “smart”, there should be no issue with leaving it on all the time keeping the battery fully charged and ready to perform. Lead acid batteries have a life normally measured in cycles – each time you discharge and then recharge the battery, this is one cycle. To extend the life of a battery you should limit the number of cycles that take place when they don’t need to. There is of course many more factors involved than this – but this is the most important.

  11. Mal Says:

    G’day Gavin,

    I’m about to replace my 4 x 12v x 250A/Hr (budget priced) gel batteries, after only 12 months, and have been told that the most likely cause of their demise was my early morning caffeine addiction and the use of a 2400 watt electric kettle to cater to this need. I thought I was looking after these batteries pretty well as, even after three boilings of the kettle in the morning, I was only using 10 to 15% or 50 to 75 of my available 500A/Hrs @ 24 volts, and it was extremely rare that the 1Kw of panels didn’t have the batteries topped up by dark, the lowest the batteries ever got was 75% after three cold cloudy Victorian days. I have also read, after hours of interweb research, that uneven cable lengths may have been partly to blame as, even though I’ve used 35 square mm from both pairs of batteries, they are all different lengths. With the new batteries I’ll be tidying this up by changing battery placement and running even length 35 sq mm between and from the two pairs into single 80 sq mm for the run to the inverter charger, although these cable sizes are probably totally unnecessary if I can’t boil the kettle.
    As I’d like to see your thoughts on this I’ll list a few system details below.


    Batteries 4 x 12v 250A/Hr hooked up series/parallel for 24v Standby 13.5 – 13.8v, Cycle 14.4 – 15v.
    New batteries will be (even more budget priced, due to necessity) 4 x 12v 220A/Hr AGMs
    Juta CM5024Z set to Float at 27v and Charge at 28.8v.
    Panels 4 x 24v x 250watt mounted flat above bus roof.
    3kw inverter/70A charger, charge could be a bit overrated.
    1kw inverter
    30A 24 to 12v reducer, runs led lights, tvs, wireless, gas hws etc
    LinkLite monitor, probably the most important part of my system, thanks Gavin.
    Griffinhome has not seen electricity from the grid since completion and the generator has only been needed since the
    batteries started to die.

  12. Hobo Says:

    Interestingly this is very similar to the gel battery setup we had in our motorhome – our batteries failed after just 26 months. We were told (and the manufacturers specs stated) that they had a design life in excess of 7 years – ours were never mistreated. After talking to a number of manufacturers and their technical staff we feel that the rough roads may have been the cause. While it is true that gel batteries are not ideal for supplying very heavy loads (2400w @ 24v = 100amps – not massive), I would doubt that this was the cause. I also doubt that the cable lengths can be blamed – this may have caused a very slight imbalance during heavy discharge, but I can see that causing a failure after just 12 months.
    The 70amp charger should be just fine for AGM batteries – these are able to accept much higher charges than GEL or flooded batteries.
    I’d be interested in the brand and country of origin of your failed GEL batteries.

  13. Mal Says:

    G’day Gavin,

    Thanks for your reply. I’m now wondering if the vibration from an out of balance tail shaft over a couple of thousand kilometres, most of which were on western NSW roads, could have been the culprit. The problem is, I’m not sure exactly when the batteries started to fail as, even though I kept a close eye on amps input/output and state of charge, I never took a lot of notice of voltage. I do vaguely remember that not long after driving back to Vic, before Christmas, the thought crossed my mind that I hadn’t seen the monitor flashing “Full”, even after a clear summer’s day with minimal load.
    The batteries are branded Toyama, and are Chinese made as is most of my setup(needs must situation).

    I’ve been trying to remember when I last saw the monitor flashing “full” and I can’t recall that happening any time since arriving here. Most days it indicated the batteries were at 100%, but now with no charge and the batteries at 24.3 volts, the monitor is showing 98.8%, so I am starting to think you may have hit the nail on the head and the problem started six months ago and I’ve only noticed it in the last couple of months as the batteries got bad enough to be a problem. Would this be similar to your experience when your’s failed?

    I’m not sure at this stage what brand the new batteries are as the details on the website don’t match the brand name on the pictured battery, but if they are as in the picture they have an Australian website and I will email them about what sort of loads I can safely draw and get back to you, if I get a reply. I can’t recall, after a lot of reading on the net, anyone giving any warnings about high amperage loads causing battery problems, besides, I can give up my electric kettle, but the microwave I regard as an absolute necessity.

    It’s probably a bit more than a coincidence that my current battery setup is similar to yours, and the purchase of AGMs this time is solely to do with price ($399 each for 12v 220A/Hr @ 20hr rate, needs must, again) and nothing to do with the failure of the Gels.

    One thing, I think, I should have done different was to have the electrician install a switch to isolate the inverter/charger from the 240v system mainly because, with the dying batteries struggling to hold 24v, if I want to power the microwave with the generator, the inverter/charger is trying to put 40A (960watts) charge into the dying batteries which doesn’t leave enough generator capacity to run the microwave, in which case the breaker on the generator trips out, the inverter takes the load, the battery voltage drops, the 24v to 12v reducer doesn’t have it’s required 23.2v so it trips out and the lights go out.

    Note for anyone with a
    Voltech BBC-3140 DC-DC Charger/Voltage Reducer/Charge Equalizer. The book that came with mine about 12 months ago states that the “Input low voltage disconnect” is 21 volts, after sending mine back to be repaired/adjusted I was informed that it is working fine and that the disconnect had been upgraded(?) to 23.2 volts, also the auto reset doesn’t happen until 25.2 volts, not 24 volts, as in the book.


  14. PaulnSans Says:

    Hi Gav

    Just a quick query on terminology in re; Depth of Discharge.

    Noting though that this divergence of terminology in no way affects the validity of the points you’ve raised above.

    I’ve always understood DoD to refer to the actual discharged energy and not the remainder energy. That is, a DoD of 66% means you’ve discharged the battery by 66% of their full capacity such that only 34% of the original full charge remains. Hence, when talking about say a 100 percent depth of discharge (DoD) using a 100aH battery we’re talking about full depletion of the full 100aH whereas I get the impression that you’re saying above that if we use NONE of the battery then the depth of discharge is 100%. I’m not sure that that is correct.

    Manufacturers prefer rating the batteries at 80 percent DoD, meaning that only 80 percent of the available energy is being delivered and 20 percent remains in reserve. Hence, why I am getting hung up on this.

    However, as I said, it’s not a critical point, just one of those niggly little technical points that dicks like me get stuck on … having nothing better to do in our miserable little lives 🙂

  15. PaulnSans Says:

    Hi … me again

    On a separate point and following on from your recommendations, I’ve gone and scored myself a Victron Battery Monitor to go into this battery box:

    Interestingly I noted that they sell the VBM’s but that for some strange reason, they choose to still stick with the Volt Meter (albeit a digital one – which we now know is still about as useful as tits on a bull courtesy of your teachings 🙂 within the battery box!

    JTS are a good lot. They supply good products and definitely get out there. I’ve seen his stuff on the 4WD show and on 4WD’s actually in the scrub (noted in my hunting magazines) – so eastern states hunters use his gear out bush!!.

    Yet they don’t seem to push the use of battery monitors with their solar setups and when I inquired about installing one in the battery box for me in lieu of the volt meter they actually didn’t have them on hand to install (ordering them in only if they needed to) and weren’t sure if they’d even fit.

    So a little disappointing – that a specialist 12Volt business (they used to be Jamies 12 Volt Shop) still runs with the notion of relying on internal voltage readings rather than a monitoring option especially given that the camp battery will always be powering a fridge at least and so a proper valid reading could not be reliably taken!!

    A point that ought to be noted about camp battery readings!!

    Cheers Gav

  16. Hobo Says:

    Interesting. It is not that volt meters are completely useless – it is more that they are useless for determining the remaining capacity of a battery. Two things that a volt meter does quite well…
    1. Tell you that the battery is under charge (v >= 12.5)
    2. Tell you when the battery is really flat (v <= 12v) With a little more experience you can tell other things as well - for example from the voltmeter that is on my dashboard, while driving I can tell what the alternator is doing and what stage the solar regulator is up to.

  17. Hobo Says:

    Always a confusing point – to be honest, I don’t know the correct use of the term DoD – I always back up the term by explaining what I mean. eg “40% DoD – that is 60% remaining in the battery”.
    Wikipedia says that a DoD of 100% is an empty battery and a DoD of 0% is full – sounds right to me.
    The opposite of DoD is SoC – state of charge. In this case a SoC of 100% is full.

  18. Paul Camplin Says:

    Hi! I have enjoyed your article, and have a question. In our motor home I have four battery charges, two 240v -12v 15 amp that I use when I am running my generator, if connected to mains then I turn one off, a single 120 watt solar panel and regulator and a Voltech DC/DC Charger that only works while the engine is running.
    These units have different specs for float and absorbtion.
    240v/15a charger. Absorption is 14.4v float is 13.6v Recond is 15.8v
    OZ charge Solar. Cut out Control 14.5v cut in control is 13.6v
    Voltech DC/DC Absoption is 14.1v Float is 13.2v
    My question, is there conflict between the sola and DC/DC chargers when both are operating. When I get to my destination after several hours drive I don’t think my batteries are as charged as I feel they be?

  19. Hobo Says:

    Hi, the short answer is yes … with a number of different charge sources you will be getting a very confused level of charge. For example one will be trying to get the battery to absorb (constant voltage) at say 14.2v, while another is trying to bulk charge (constant current). It wont do any harm, but most of the time (unless your battery bank is very flat) only one charger will be doing all the work.
    A good quality battery monitor would go a long way in helping you see the real charge status of the batteries.

  20. Rosco Says:

    Dear Hobo.

    I live in a small rural town and often get requested to rectify faults in various installations in caravans, mainly television, however on one occasion I was asked if I could find out why a caravaner was having trouble with his solar cell panel overcharging his battery. It was a simple system with one 120 watt (I think) panel through a 12/24 auto detect regulator and also a simple 240v battery charger both connected together. He told me he had already killed one battery and felt this one was going to head the same way. I had not had much experience with these particular systems but at his request told him I would “have a go for him”. He was desperate. when I arrived the flooded battery was boiling well. It wes early afternoon. After some time I realised the regulator wasn’t regulating and it appeared to be treating the battery as a flat 24V one. I turned off the 240V charger but it still wouldn’t regulate. Eventually I tried covering the cell with a towel, leaving the 240V charger off, discharging the battey a little, then removing the towel. HEY PRESTO, it started regulating again, I monitored it whilst putting load on the battery and letting it charge again, and it all worked fine. My belief is that the 240V charger was creating a ripple? to make the solar regulator think it was a 24V battery. I advised the custumer to leave the 240 charger off, and seek the advice of a professional caravan electrician when he could and parted from a VERY happy customer after being suitably rewarded for my efforts and perserverence.
    I now ask you, have you ever expeienced this happening, and if you are running solar, why would you want 240V charging anyway. He only used the battery for TV, LED lighting and water pump. The rest of his things ran straight off 240V. Jugs microwave etc. To me it is just something else to go wrong. If it was my van, I would have added an extra solar panel or even two to ensure the battery was always topped up. There was no shortage of space on his roof.
    Your reply would be appreciated.
    Kind regards.

  21. Hobo Says:

    Hi Rosco,
    I have had the same issue you mention in some dealings with a motorhome in Alice Springs years ago. The regulator was incorrectly deciding that the system was 24v. It is for that reason that I have never liked regulators that “auto select” preferring to set the system voltage myself.
    I agree that ideally 240v charging should not be required – but then there are those cloudy weeks … 🙂


  22. Rosco Says:

    Hi Hobo.

    Thanks for your reply.

    Did you reset that system in a similar fashion to what I did or did you have to resort to something else.

    We get a lot of tourists throught here now, so it is very handy for me to know the tricks to resolving different problems in their systems. Many approach me for a solution, and quite a few do not have a user manual for their system. The internet is a great resorce in that case.

    I am like you in regards to “auto selct” regulators.


  23. Hobo Says:

    In our case the regulator only looked at the voltage to determine the system voltage at startup. By simply not having the charger on when the regulator was first connected we avoided the issue.


    Hi Hobo,
    Have just been told that the two DCB on my three year old caravan needs replacing, they used a voltmeter to come to this
    decision, I had also just had Redarc charger added to the vehivle as I was told that there was insufficient voltage getting through the Anderson plug.The problem is that I can only bush camp for one night and then go to a park to recharge the batteries, we have the fridge on 12v when travelling.How should the DCB be tested to give a true Indicationof the life left in the batts.
    The charger on the caravan shows the 240 v input/state of charging and another meter shows what the caravan is drawing at 12v.


  25. Hobo Says:

    Most battery shops use load testing to gain an understanding of the state of a battery. This involves fully charging them (normally slowly) then applying a known load and monitoring the terminal voltage over time with the load attached. They stop the test when the terminal voltage gets to a point then calculate the total AH delivered and compare this to the specs of the battery. If the test you had performed simply tested the voltage across the battery, unless it was VERY low, is meaningless.
    If the only means of charging is from the vehicle alternator, then the RedArc step-up charger will be of great value as it will fully charge the battery regardless of the distance and voltage drop to the alternator.

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