Updated November 2016

Knowing how to tell caravan battery charge is not easy. Get this wrong and good batteries are scrapped and bad ones retained. Here’s why, and how to tell.

Knowing how to tell caravan battery charge is a particular problem with the deep cycle lead acid batteries most caravans and motor homes still use. These batteries have a time lag (of 24 hours or more) between drawing a heavy load and the voltage you measure across them. Even then the measurement is likely to be only with 10%. Because of this any instant voltage measurement of a deep cycle battery has virtually no meaning. As a result perfectly good batteries are replaced. Worn-out ones are retained. The main problem (of how to tell caravan battery charge) is not knowing the above. (This is less of an issue with AGM and starter batteries.)

Collyn in his office

Pic (of the author) copyright caravanandmotorhomebooks.com

Why the time lag

A lead acid battery stores energy via an electro-chemical reaction between lead plates and an electrolyte (sulphuric acid diluted in water). In effect the stored energy is within that electrolyte. Its specific gravity (density relative to distilled water) changes according to the state of charge.

The voltage measured across a well rested battery reflects that specific gravity. But if a deep cycle battery has just supplied high current, the specific gravity of that acid is now much lower in and adjacent to the lead plates. And it is across those plates from which any voltage measurement is taken. It can take as long as 72 hours for the effect of charging and heavy discharging to be evenly distributed. It is only then that a voltage measurement has true meaning.

The greater the individual battery capacity the longer the rest time required. Despite this, many people make spot checks of deep cycle battery voltage.

An almost worn out deep-cycle battery may show a close to full-charged voltage when charging even begins. But what’s being measured is the voltage from the battery charger. It is only high because the battery can no longer accept it!

The table below shows the typical relationship between voltage and remaining charge. It is only meaningful, however, for deep cycle batteries that are rested – ideally for three days. It’s otherwise only of academic interest.

battery state of charge

Typical rested voltage of a lead acid deep cycle battery. Pic: original source unknown.

(Starter batteries have a far larger number of also thinner plates. This enables them to supply high currents and to recharge rapidly. But even with these the battery needs to rest for about 30 minutes before its voltage truly reflects its state of charge.)

Lithium-ion batteries

A similar problem (but from a totally different cause) is with lithium-ion batteries. In typical caravan and motor home use these batteries only fall by a typical 0.1 volt from 90% to 10% charge – say 13.0-12.9 volts (or on light loads 13.1-13.0 volts) They are damaged (or wrecked) if fully discharged. They also need accurate control of charging voltage and the minimum  state of charge (plus individual cell monitoring and balancing).

How to tell caravan battery charge 

For all but starter batteries there’s only reliable way of knowing the battery’s state of charge. It does not give an exact measure, but is close enough for caravan and motor home purposes.

How to tell caravan battery charge is done the same way you keep track of your money: count what comes in, deduct what goes out (and the bank’s charge for storing it!). What’s left is what you have. Caravan/motor home battery happiness is making sure it’s always in credit. (It’s not unlike the fuel gauges in today’s cars that show instant fuel usage as well as totals).


                                    Energy monitor Victron good                                 batter monitor xantrex web

Typical energy monitors. Pix: Victron Energy, Xantrex.

 

This way of charge measuring is built into some dc-dc alternator chargers and the more up market solar regulators. It can also be done by stand-alone energy monitors located where they are readily seen. The best known is the Xantrex. They are also sold also under various other brand-names. Energy monitors are readily installed by anyone with a reasonable knowledge of caravan and motor home electrics. The monitor is (easily) programmable for battery nominal voltage (e.g 12-24-48 12 volt, type (lead acid deep cycle, AGM, gel cell) etc. Most recalibrate themselves from time to time. These units are typically accurate within plus/minus 5% or so. For LiFePO4 battery monitoring consult the battery supplier.

Full details of every aspect of battery charging, energy monitoring etc is covered in the constantly updated Caravan & Motorhome Electrics. It was written for the non-specialist, but has also become the main reference for auto-electricians. For energy monitoring of solar see also Solar That Really Works. Solar for homes and properties is covered in my book Solar Success.

Virtually every aspect of caravan and motor home usage is covered in the all-new Caravan & Motorhome Book. Camper trailers are more specialised – these are covered in detail in the Camper Trailer Book. For details of the author Click on Bio.

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