Monitor your batteries for increased life

Xantrex Battery MonitorMost caravanners and motorhomers rely on battery power to some degree or other. If you have been listening to the experts, you will know that the best way to ensure a long and productive life for your expensive deep cycle batteries is to pay close attention to how the batteries are charged and to what level they are discharged.
Discharging a battery beyond 50% is almost as destructive as dropping it!
Whenever I talk to people about how full their batteries are, I almost inevitably get told “the batteries are at 12.2 volts” or something similar. Being trained as an electrician I not only know that battery voltage is no indication of state of charge, I also understand why. Trust me, if you want to know the true charge state of your batteries, don’t reach for a volt meter; you are wasting your time.
A couple of years ago I purchased and fitted a Xantrex battery monitor. This device has no other purpose in life other than to constantly monitor your batteries state of charge and state of health.

So how does it work?
If I asked you to accurately tell me at any point in time how much water was inside a partly filled container (of a known capacity), there are a number of ways that you could achieve this. One  method would be to fill the container to its capacity, then carefully measuring all water flowing into and out of the container.  You could then, with some degree of accuracy, tell me at any point in time, how much water was in the container. As it happens, this is exactly how the Xantrex battery monitor works. In the electrical world there are some other factors to be considered and calculated – but by enlarge, the principal is the same.
Water Containers don’t normally shrink over time, battery capacity does. The Xantrex unit not only takes this into consideration but it is also able to report on this shrinkage (known as the charge efficiency factor).

This little unit sells for around the $400 mark. The batteries in our motorhome are now over 5 years old and are showing very little signs of aging. I believe this is, to some degree, as a direct result of our ability to closely monitor their charge state using the Xantrex Battery Monitor.

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14 Responses to “Monitor your batteries for increased life”

  1. Motorhome and Caravan News Australia » Blog Archive » Getting the electrics right Says:

    […] a volt meter! A correctly configured PL40 can tell you – a properly installed and configured Xantrex battery monitor will tell you… a volt meter will […]

  2. wayne Says:

    Hi Ive recently bought a fold up solar panel. Set it up by soldering 12 volt female end to it. ( cigarette lighter type ). Plugged it in to my engel batt pack. Run the engel. Overnight it drained the battery to the point that the fridge stopped. Normally the fridge would run for 2-3 days on fully charged battery, which it was. Am I doing something wrong. Any help would be appreciated. Wayne

  3. Hobo Says:

    Hi Wayne,
    There are a few things we need to check…

    1. Was the battery fully charged before you started?
    2. Check that you have the positive from the panel going to the positive of the battery.
    3. With no regulator in the circuit (unless there is one in the panel arrangement) – you risk damaging the battery by over charging it.
    4. Be sure that there are no electronics (inside the battery pack)connected between the panel and the battery terminals.
    5. An ammeter will quickly tell you what is happening with the battery both when the panels are connected in the sun and at night. It is possible that the battery is discharging back through the panels (although unlikely to be a large amount) – again, an ammeter will test this.

    See the article on testing solar for information about using an ammeter.

    Please let us know how you get on.


  4. wayne Says:

    hey gavin
    ive worked out my problem thanks to you
    the engel bat pack has electronics attached to it
    im now using 1 of my deep cycle batteries
    attached to my solar panel
    the fridge has been running for 4 or so days and the battery is holding about 12.5 volts
    thanks for the tips
    happy travels

  5. Sharyn and Peter Says:

    Gday Gavin. We are at the stage of choosing a fridge for our bus/motorhome under construction.Have been deciding between 3 way or 12 volt 240 watt.We hope to have between 400 and 500 amp hour batteries and will have solar panels plus a kipor 2.6 kva generator as a backup.We a leaning towards the 12volt option mainly because they are easier to position in the bus venting and heat are not such issues as with 3 way i hope i am right with that information as that is what i have been advised.We hope to get a 200 ltr fridge do you think the battery pack will be sufficient to operate the fridge as well as every thing else in the bus. Just looked at your bus we cant go 24 volt because we already have 12 volt water pumps.Our hot water and cooking will be gas along with either gas or diesel heating (undecided).You advise would be much appreciated on batteries and the amount of solar panels (watts) required to keep the batteries charged. Await your reply. Peter and Sharyn. Ps any advise on the type of batteries would be great.

  6. Hobo Says:

    Hi guys – thanks for the question. When we first purchased our motorhome (some time ago now) it had a “three way” fridge (I prefer to call them gas fridges – as that is really what they were designed to run on). IT worked ok when the ambient temperature was below about 30 deg – but above that it basically stopped. It used a 9kg gas bottle in less than 3 weeks (at outback prices – that’s about $60/mth). Dumping that gas fridge was the single best thing we have ever done to our motorhome. I am sure that new gas fridges are much better – but I have spoken to very few owners of gas fridges (who travel in the tropics) who are happy. My recommendation would be a 12v/24v Dansoss based compressor fridge.
    With regards to the question about battery capacity – for the fridge it is more about solar capacity. 400Ah – 500Ah is plenty of battery. Having plenty of solar is far more important.
    Our electric fridge is 220ltr – it uses (very approximately) 280 watts of solar to keep it going in 95% of conditions. You will need to calculate the usage for your other electrical needs to come up with a total solar requirement. There is an article on this site about that.
    AGM batteries are the best choice for both economical and practical reasons – they are cheaper and lighter for a given USABLE capacity (but quite a bit more expensive to buy).
    I hope this helps –

    BTW – we would love to see your motorhome in our growing “other rigs” section of the website –

  7. spike Says:

    do u know how long a 12volt new 12/240 v in a new 2012 avan applause600 21 ft. motorhome with 95ah house
    battery would run including led lites[apx 1hr per day max,12 v water pump 10 mins per day] and the rest on gas
    so as not to flaten house batt. below apx. 50 % thanking you spike

  8. Hobo Says:

    Hi Spike,

    The formular is

    For each item, take the total current and multiply this by the number of hours the item will run per day. This will give you amp hours required per day for that item. Total all of these to give total amp hours required per day. I would add about 10% to account for losses.
    A 95ah battery will provide about 40ah (less if discharged quickly) so it is a simple calculation to determin the number of days that the battery will run the caravan.

    You could add a couple of solar panels – then the answer would be “forever”.

  9. Jay Says:

    Hi Gavin,

    After draining my 100Ah AGM battery with a 12V 72watt – 158 liter fridge . I am upgrading to 2 x 100Ah AGM battery & 2 x 200watt solar panels. How long with and without sun should this setup run the fridge on its own?

    Nice thread you’ve got going here.

  10. Hobo Says:

    This is difficult to say with any real accuracy. Lots of factors to consider…
    – How long do you want the batteries to last? (deeper discharge = shorter life)
    – Winter of summer?
    – What part of the country?
    – Ambient temperature?
    You get the idea.

    If you are happy to discharge the batteries to 50% while parking in a cave (no sun) assuming a 33% duty cycle on the fridge, the batteries will last about 50 hours if nothing else is drawing power. (72w @ 12v = 6amps X 33% = 2 amphours each hour – with 100ah to play with you will have 50hrs). This will be much less in high ambient temperatures. I would expect 400w of solar to run this fridge indefinably under most conditions.

  11. Vicky Moore Says:

    My husband insists every time we pull up for petrol and toilet break, a short lunch or shopping that we turn our fridge off while we are doing this, We have a 185 ltr 3 way Thetford fridge, we have 2 !00amp hr AGM batteries, he feels it is necessary to do this so as not to drain them. We know a few people who do not do this, do you think it is a good idea or a bit over the top. thank you Vicky

  12. Hobo Says:

    Now just to be clear, I’m not just “siding with the boys” 🙂
    I actually think it is very good practice to do this. These fridges use a lot of power when operating on 12v (about three times as much as the equivalent sized compressor fridge). In reality, a few minutes running with the engine off will not do any harm – however it is very easy to forget to turn it off if you get distracted and end up staying longer than planned. If this happens the end result could well be a flat battery. The simple “stop the engine = stop the fridge” rule seems to me to be a good way of preventing this. Remember the old Australian saying “tiz better to have warm beer than a flat battery”.



  13. les k Says:

    top read mate ,,,,, just confirming similar to Jay , but I want to run a 240v 75 watt led Median 81cm tv thru a 12 v/240 v invertor off a 220AH battery with 2x 200watt solar panels .what would be the formula for this please …thanks les

  14. Hobo Says:

    Hi Les,
    I’m not sure what you are trying to work out here.
    If you are asking if this will work, we would need to know how many hours you plan to run the TV each day.
    The TV will use 6.5ah per hour plus inverter losses (lets say 7.5ah/hr)
    Your 400w of solar will generate (about) 125ah per day (depending on where you are in the country). Assuming that the TV is the only load, you should be able to run the TV for about 16 hours per day and still keep the battery fully charged.

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