Updated May 2017

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 deep cycle lead acid batteries. These batteries have a time lag (plus 24 hours) between heavy loads and their subsequent voltage. Even then, measurement is likely to be only within 10%. Any instant voltage measurement thus has little meaning. As a result perfectly good such batteries are replaced. Worn-out ones are retained. Not knowing how to tell caravan battery charge is the problem. It is less of an issue with AGM and starter batteries.

Collyn in his office

Pic (of the author) copyright caravanandmotorhomebooks.com

How to tell caravan battery charge – why the time lag?

An electro-chemical reaction between lead plates and an electrolyte enables lead acid batteries to store energy. The electroyte is sulphuric acid diluted in water. In effect, that electrolyte stores energy. Its specific gravity (density relative to distilled water) varies with the state of charge.

Specific gravity reflects voltage across a well rested battery. The specific gravity of deep cycle battery that has just supplied high current is consequently lower close to its plates. Voltage is measured is that across those plates. It accordingly takes a day or more for charge to evenly redistribute. Only then, does voltage measurement have true meaning.

The greater the individual battery capacity the longer the rest time accordingly required. It is, therefore, meaningless to check deep cycle voltage directly after charging or discharging

It’s when charging begins that an almost worn out battery shows close to full voltage. But that measured is battery charger voltage. It’s high only because the battery cannot accept it!

This table shows typical relationship between voltage and remaining charge.

battery state of charge

A lead acid deep cycle battery’s typical rested voltage. Pic: original unknown.

Starter batteries have a far larger number of thinner plates. This enables them to supply high currents and to recharge rapidly. Yet, before voltage truly reflects charge, even these need some rest.

How to tell caravan battery charge – lithium-ion batteries

Lithium-ion batteries have a similar problem (but a different cause). In typical RV use these fall only 0.1 volt. Typically from 90% to 10% charge. This is about 13.0 to 12.9 volts or, on light loads 13.1 to 13.0 volts. If fully discharged, they are damaged. Or wrecked – as some claim. In addition, they need accurate control of charging voltage and minimum state of charge. Furthermore, individual cell monitoring and balancing is mandatory.

How to tell caravan battery charge – energy monitoring

 There’s only one reliable way of knowing the state of charge for all but starter batteries. It doesn’t, however, give exact measure. But it’s close enough for RV purposes.

It works much as you track money. Count what comes in. Deduct what goes out (and the bank’s charge for storage). That left is what you have. Ensuring it’s always in credit equally assures battery happiness. It’s like fuel gauges that show instant 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. It is also used in up-market solar regulators. Some have remote monitors that you locate where readily seen. The best known is Xantrex. It is also sold under other brand-names. Energy monitors are readily installed, and furthermore (easily) programmable. They must be set for battery voltage, battery type, e.g lead acid deep cycle, AGM, gel cell. For LiFePO4 monitoring consult the battery supplier.

Energy monitor accuracy is typically accurate within plus/minus 5% or so. They recalibrate automatically.

How to tell caravan battery charge – further information

See Caravan & Motorhome Electrics for more. For solar energy monitoring see Solar That Really Works. Solar Success is for home and property systems.

Every aspect of RV usage is covered in the Caravan & Motorhome Book.  For camper trailers see the Camper Trailer Book. For author info Click on Bio.

If you find this article useful, please Link to it if discussed on a forum.