How to tell caravan battery charge – energy monitoring explained

by Collyn Rivers

Measuring caravan battery charge is particularly complex for deep-cycle lead-acid batteries. These have a long time lag (24 hours plus) between heavy loads and subsequent voltage. Even then, measurement maybe only within 10%. Instant voltage measurement has no meaning. As a result, perfectly good batteries are replaced. Worn-out ones are retained. Not knowing how to tell caravan deep cycle battery charge is the main problem. It is less so with AGM and starter batteries. And easy with lithium batteries.

Pic (of the author) copyright rvbooks.com.au

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

An electrochemical reaction between lead plates and an electrolyte enables lead-acid batteries to store energy. The electrolyte is sulphuric acid diluted by water. In effect, that electrolyte stores energy. Its specific gravity (density relative to distilled water) varies with a battery’s state of charge.

Specific gravity reflects voltage across a well-rested battery. That of a deep cycle battery that has just supplied high current is lower close to its plates. Voltage is often measured across those plates. The charge needs a day or more to evenly redistribute, so only then can measuring be meaningful.

An almost worn-out battery may show close to full voltage as charging begins. That being measured, however, is battery charger voltage. It is high because the battery cannot absorb it.

This table below shows approximate relationships between voltage and remaining charge.

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

Starter batteries have many and thinner plates. This enables them to supply high currents and to recharge rapidly. Yet, before voltage truly reflects their charge even these need a few minutes rest.

How to tell caravan battery charge – lithium-ion batteries

Lithium-ion batteries have a similar problem, but for a different cause. In typical RV use these fall only 0.1 volt from 90% to 10% charge (typically 13.0 to 12.9 volts).

Fully discharging these batteries may damage or even wreck them. In addition, they need accurate control of charging voltage and minimum state of charge. Furthermore, individual cell monitoring and balancing are mandatory.

How to tell caravan battery charge – energy monitoring

For all but starter batteries there’s only one reliable way of knowing battery state of charge. It does not give an exact measure but its close enough for RV use. 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.

  

Typical energy monitors. Pix: Victron Energy, Xantrex.

Such energy monitoring is built into most dc-dc alternator chargers. It is also included in up-market solar regulators. Some also have remote monitors. The best known is the Xantrex.

Energy monitors are readily installed. Furthermore, they are easy to programme. Moreover, energy monitor accuracy is typically plus/minus 5%. 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.