How Charged are my Motorhome/Caravan Batteries?

No matter whether you are a “The glass is half full” kind of person or a “The glass is half empty” type, you will agree that one advantage of drinking from a glass is that you can see just how much of the liquid remains. Sadly our motorhome and caravan batteries are not like that – there is no way we can tell by simply looking at the batteries how much capacity remains.  In fact, it is surprisingly difficult to say with any degree of accuracy, just how full your batteries are.

If you have flooded, lead acid batteries (not AGM or GEL type), you could dip a hydrometer into the battery acid and from the specific gravity reading get a fairly good idea of the current state of charge. This is of course not possible with any of the now very popular sealed types.

Motorhome Battery

What about measuring the battery voltage?

About four times a week I get an email or meet a traveller who has a question or an issue with their electrical system. They proceed to tell me that their batteries are at x percent because the battery voltage is at y.yy volts. Thus about four times each week I repeat the following speech …

(lets all say it together)

Battery terminal voltage is almost of no use when trying to estimate the state of charge of your batteries

Why does that well practised speech contain the word “Almost”? Well I say “Almost” because there are two (and only two) exceptions to that rule:

  1. If your battery voltage is reading very low(less than say 10.5 volts) with little or no load having been drawn from the batteries for 30 minutes, then the batteries are about as flat as you will want to take them.
  2. If your battery voltage is reading more than 14 volts for 10 minutes when the batteries are under charge (by solar or other method of charge) AND the charge current is not huge (ie. less than 10 amps), then it is reasonable to assume that the batteries are very close to fully charged.
(Note – both of these situations assume batteries and connections in good condition – strange things happen when the batteries or terminal connections are faulty).

Other than these two extreme ends of the scale, battery voltage will tell you as much about the batteries state of charge as the current colour of the sky.

If you have a voltmeter attached to the system (or worse still a simple LED type battery indicator) and are using this like a vehicle fuel gauge, you are making assumptions on false information.

So how do the so called “smart chargers” and smart solar regulators know when the batteries are full or in need of charge?

The short answer is that they all use the good rule of thumb … “if it is not full … charge it”. Smart chargers and solar regulators watch for the prolonged high terminal voltage that signals that a battery bank is full. At this point they stop or reduce their charge. They then watch for the battery voltage to drop below a preset point and resume charging. These smart charges do not know HOW flat the battery is, just simply that it needs charging.

So now we know how NOT to measure the state of our batteries – how CAN we get this information?

Apart from dipping a hydrometer, there is only one accurate way of determining the current state of of your batteries.

A number of manufactures have produced microprocessor based battery monitors. These tend to be expensive ($200 – $500) and require far more work to install than a simple voltmeter. They contain both a voltmeter and a current meter and their principal of operation is as follows:

  • They require that you program into them the total battery capacity (in amp-hours)
  • They then watch for the signs that the battery bank is fully charged (normally prolonged high terminal voltage and small charge current). At this point they reset and read 100%.
  • They then monitor all current that flows into and out of the battery and simply add and subtract capacity as appropriate.

Example :

The battery voltage and current conditions indicate to the monitor that the 100 amp-hour battery is fully charged – the monitor reads 100%.

The monitor “Sees” ten amps being drawn from the battery for one hour.  Ten amps for one hour = ten amp-hours. It can conclude that the 100 amp-hour battery now has 90 amp-hours remaining  and thus displays 90%.

This process of “watching” current flowing into and out of a battery is the ONLY reliable method that can be used to accurately determine the current state of the batteries.

A correctly wired Plasmatroncs PL series solar regulator includes this function. The Xantrex company also make a very good stand-alone battery monitor that includes an advanced version of this function that makes it both self-learning and very accurate.

Got a question or a comment about motorhome or caravan electrics? Perhaps you have a comment about this article. If so, please feel free to email me (email address can be found on our contacts page) OR use the comments box below.

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4 Responses to “How Charged are my Motorhome/Caravan Batteries?”

  1. ERic Markham Says:

    Hi Gav
    With our $ being so high have been thinking deriosly at buying a Xantrex monitor from US. as they are now under $200. Do you know if you need their special wireing harness or are they fairly easy to wire and install. The harnesses are quite expensive and much too big for my planned installation. Any feed back would be great. =Cheers Eric

  2. Hobo Says:

    Hi Eric,
    As long as the unit comes with the shunt it should be very simple to fit. I did not get a wiring harness with my unit and it was no issue.
    These are great units and I highly recommend them.
    Gavin

  3. geof Says:

    I have a 4 yr old motorhome with 3 100amp deepcycle batteries on continual solar topup. they show a reading of 14.3v on the monitor (a smart monitor that tells me amps going in/being used etc). However when the fuse has tripped and the invertor kicks in the batteries seem to lose their charge quite quickly (fridge on AC, lights on (led’s) & TV on). How can I tell if the batteries are worn out – I’m told using the “thumper” as one would on a vehicle battery is not a good idea with deep cycle batteries as it can warp the plates.
    Any ideas?

  4. Hobo Says:

    Hi,
    The first thing I would do is determine IF you really have an issue. It is quite normal for the battery voltage to drop when a load is applied to the battery – the bigger the load, the bigger the drop. Battery voltage is a very poor indicator of the state of your batteries. (Take a look at the article on this site about that).
    I would check the current being drawn (if the fridge is a three-way, it will be a large draw) then allow the batteries to carry the load for a few hours and then check the battery voltage.
    It is difficult to provide a guide, but so long as the voltage does not go below 12v when you draw 20amps for 6 hrs the batteries are probably ok.
    If you have access to a clip-on ammeter it can also be useful to check how much of the load each battery is supplying – they should all be the same (clip onto the leads connecting each battery). A major difference suggests a single dead battery.

    If the batteries are not sealed, use a hydrometer to check the specific gravity and clarity of the fluid of each cell – they should all be nearly the same and clear.

    Please let us know how you get on.

    Gavin

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