Why are my motorhome / caravan batteries going flat?

As we travel, we often meet people who are in caravans and mobile homes. It always surprises me how many people continue to have issues with the electrical systems. In most cases the conversation starts with “why does my fridge cut-out during the night” or perhaps “my caravan lights go very dim by about 10pm”.

Almost inevitably, people have unreasonable expectations of either their solar panels or batteries (or even more likely both).

In this discussion I would like to focus on the common misconception regrading battery capacity.

Deep Cycle Batteries

Deep cycle batteries are sold by capacity and are normally rated in amp-hours (Ah).

When we buy 500 grams of butter, we rightfully expect to be able to spread 500 grams of butter on our toast before discarding the container. It would be reasonable to expect a 100 amp hour battery to deliver one hundred amp hours – one amp for one hundred hours OR ten amps for ten hours.

Sadly this a long way from the truth. The number of amp hours you can extract from a given battery is dependant on the following factors:

  • The battery’s chemistry – for example – common lead acid batteries will deliver far less energy than AGM or GEL batteries for the same amp hour rating.
  • The batteries age – as batteries age they become less able to deliver the same capacity.
  • Your willingness to shorten the life of the batteries by deeply discharging the battery.
  • The chargers ability to fully charge the battery.

Here is an example:

John purchased two 60Ah deep cycle batteries for his caravan. He charges them from a single 80 watt solar panel and from his vehicle alternator when travelling.

Why is John’s electric refrigerator cutting-out at night even after driving for 3 hours?

Two times 60Ah = 120Ah

Now most deep cycle batteries will deliver less than %50 of their rated capacity before the terminal voltage drops below an acceptable value for electric fridges.

50% of 120Ah = 60Ah

However it is very unlikely that the vehicle alternator will charge these batteries beyond 70%. So they were 30% down before the vehicle even stopped!

John only has 20% of the original 120Ah that he thought he purchased!

That is just 24Ah of usable battery capacity.

The 80 watt solar panel will produce about 4.5 amps for say, 5 hours per day. That is really only enough to keep the fridge running during the day. The batteries are not really receiving any useful charge at all.

By the time the sun goes down the batteries are already beginning to groan in pain as the load of the fridge begins to attempt to draw more from their depleted energy store.

To make this situation even worse, the life expectancy of all lead acid batteries is directly proportional to the depth that they are discharged. The deeper you discharge your batteries, the faster they will deteriorate. At this rate John will be back at the battery shop in no time at all.

The solution –

  1. Charge the batteries fully with a smart (multi-stage) charger. This will allow the batteries to get to 100% fully charged.
  2. Install additional solar panels so that there is sufficient current to both run the fridge during the day, and replace all the energy removed from the batteries during the night.

It is possible to live on solar energy – we have been doing it successfully in our motorhome for more than 6 years. You simply need to understand a few basic principals. Doing so will save both money and precious hair.

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11 Responses to “Why are my motorhome / caravan batteries going flat?”

  1. Phillip Nourse Says:

    Thank you for a very useful website providing useful information.

  2. Phillip Nourse Says:

    Thank you for a very useful website providing useful information. Can anyone tell me why 12 volt fridges are so much more expensive than 240 volt ac fridges? I can understand double the price but 5 times more??

  3. Hobo Says:

    Yes, I struggle to understand this myself. One reason may be that the compressor used in most low voltage fridges is made in Germany by a company called Danfoss. They do such a great product that they really have almost no competition. I guess that this allows them to charge a premium for their product.

    The other issue may be that when compared to 240v fridges, the market is tiny.

    I have heard of a few fridge mechanics who are doing their own conversions. One in Geroldton buys second hand 240v fridges with dead compressors and fist new Danfoss compressors. This is a very economical option. He quoted $1200 for a 250 litre fridge fully converted.

  4. Bernie Says:

    What is the current use of Lithium Battery (LifePO4) and can they be used with Gell cell together

  5. Hobo Says:

    Lithium is without a doubt the future of motorhome/caravan house batteries. The energy storage to weight ratio is many times better than our current lead based cells.
    There are a number of manufacturers that currently sell lithium “deep cycle” solutions – to buy one of these now would earn you the title “early adopter”. They are VERY expensive (up to 5 times the cost of lead based batteries) and have not been well tested over time.
    There seems to be two different approaches to producing these batteries:
    1. Use electronics inside each battery pack to make it look (from an electrical point of view) like an AGM battery. This allows for the use of existing chargers and regulators. The down side is that because the electronics are built into each battery pack, They are discarded with the battery.
    2. Allow the lithium battery to act natively – this is far more efficient, but requires a change of regulator (for solar) and charger units.

    Apart from the weight advantage, lithium also has the ability to accept a charge much quicker than lead batteries. It can also accept this larger charge up to the point where it is almost 100% fully charged (unlike lead acid batteries that need a slowly reducing charge during the absorption phase). All of this makes them ideal for use in a motorhome or caravan using solar panels to charge.
    One major word of warning – if not correctly handled, monitored and managed, lithium cells can (and do) explode in spectacular fashion. Anybody considering lithium house batteries would be wise to consider a fireproof enclosure to house the batteries.

    Watch this space over the next few years – I expect to see great things happen.


  6. John and Gill Says:

    Wow….your website is so helpful…..thanks so much…..my husband John is amazing at all things tech. but really wanted to check with other peoples very valuable insights and experiences…..especially in the costly exercise of replacing the 8 ‘house’ batteries on our newly acquired Denning bus (Destiny).
    We are setting off soon…..just around WA at first, but later all around Auz once we have settled our two boys into the new life of FREEDOM!!!! Just got to survive these last four weeks of checking and fine tuning systems, painting, cleaning, packing, stocking, sorting, selling, homeschooling, and of course getting about 3 hours of sleep a night.
    Was really laughing to discover that like us, you were from NZ. That Gavin was electronics trained before heading into IT….just like John, and that Tracey was a nurse (I was years ago but have forgotten more than I learnt LOL)….and typical of Kiwi’s you seem as ready to jump headlong into a challenge! Fantastic!!!
    Gill…and totally impressive website too guys!!!
    PS. Could John phone you sometime at your convenience to ask a few things before we head out of State…….he drives me nuts trying to tell me all about tech stuff….(he’s a lovable, geeky, but totally clever with his workshop skills type of man) …..he needs another bloke to yap about volts, power sources, generators, batteries, hard drives and well anything that has a number attached to it really!) And hey, no probs if you are too busy tho….your blogs are really informative THANKS!!! 🙂

  7. Hobo Says:

    Hi guys,

    So glad you find the site useful. It sounds like we have heaps in common. Hopeful we can meat up sometime – perhaps by a nice beach or river somewhere.
    No problem to phone any-time – always happy to talk with fellow motorhomers. Our number is on the website

  8. John and Gill Says:

    Haha…thanks for that….and keep these great articles coming…..we are nearly heading off for the first time….really exciting.
    We decided on AGM house batteries in the end and are happy with this decision.
    Have you ever done an article on solar verses wind power generation for a motor home?
    PS. I’m doing the writing cos John’s doing all the work LOL….just kidding!!!!

  9. Hobo Says:

    Hi guys,
    I did do some elevation of wind power for travellers. It does not stack up well against solar. Long story short : it is a pain to setup and if it is windy enough to produce anything – it is too windy to stay there! So solar every time (especially given the low cost of panels these days!)

  10. fran gray Says:

    Hi guys, we have our bus running totally on solar. Usually it runs very well but about 6 mths ago we started having problems. dropped into a solar shop he had a look and told us 2 of our panels had failed (Cheap chinese junk ) thus resulting in needing a new regulator and having to replace 5 x 100amp/hr Gel batteries, and remove 2 panels. as you would be aware a costly process. Now we run 650Watt of panels, a 60amp regulator (new) and 5x 100 amp/hr Deep cycle Gel batteries also new. The system has been working well until about a week ago. It is charging well but on darkness not holding charge. For example at 5pm charge was 13.6, 2 hrs later 11.9. All we have running is 12v fridge at 6amp and couple led lights and small inverter all up 7.6 amp draw. Question could we have a faulty battery, they are only 6 weeks old or possibly another faulty panel that is drawing the power back out of the system. any suggestions would be greatly appreciated. Now hubby just pulled the fuse on the panels tonight, and the charge went back up to 12.2 and the panels are sitting at 6.9volts and they are disconnected.

  11. Hobo Says:

    This is something best diagnosed by phone. Please see my contact details on the website and give me a call sometime.

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