RV Battery State of Charge

What is State of Charge ('SOC')?

It is not easy to check RV battery charge, yet get this wrong and good batteries are scrapped and bad ones retained.

The main problem is with deep cycle lead acid batteries – where electrical energy is stored as an electro-chemical reaction between a few thick lead plates and water/acid electrolyte. Following fast charging, or supplying a heavy load (e.g. a microwave oven) the reaction takes a day or more for voltage to equalise.  (It is like roasting a large turkey, the outside may be hot, but the inside almost cold).

Likewise, measuring the voltage of such batteries after charging or discharging gives no idea of its condition. A microwave oven may reduce the charge within and around the plates to 10 volts, yet ample charge is still available once the electrolyte (that in effect holds the charge) has equalised (over time). Instantaneous voltage measurement, however, shows a perfectly sound battery as useless.

An almost worn-out battery only barely accepts charge. Voltage measurement here shows the battery charger’s voltage (high because the battery cannot accept it).

As a direct result, owners reject perfectly good batteries, yet retain those worn out.

Battery voltage measurement is fine, but (for deep cycle lead acid units) only after 24 hours totally off-load. This table shows the then typical relationship between voltage and remaining charge for a lead acid battery in good condition.

AGM Batteries

The above is far less so for AGM batteries. These too have some time lag - but a voltage measurement is a reasonable indication of state of charge after 15-30 minutes.

LiFePo4 Batteries

LiFePo4 (12 volt) batteries in typical RV use vary from about 13.1 to 12.9 volts when between 95% and 20% charge. It is this close to impossible to gain even a rough idea from voltage measurement. It is done much as we keep check of our bank balance. Measure what comes in and keep check of what goes out. Deduct the equivalent of the bank's fee for storing it. The result is what you have now.

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

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

Starter batteries have many but thinner plates, enabling them to accept and supply high currents and to recharge rapidly. Much the same applies to AGM batteries. With both, a voltage reading will be meaningful after the battery has rested for an hour or totally off-load.

Lithium-ion batteries (LifePo4)

Lithium-ion batteries present a problem of very different nature. The LiFePO4 versions used in RVs maintain 13.1-12.9 volts regardless of any load that an RV is likely to apply across most of their working range. The solution here is energy monitoring.

Energy Monitoring

Energy monitoring (Coulomb counting) is the only reliable way of knowing a battery’s state of charge. It is not 100% but good enough for RV purposes. It works much as you track money. Count what comes in, deduct that going out (and the bank's charge for storing that remaining), that left is what you have.

Energy monitors rely on a current flow signal using a device called a current shunt that sends the data to the remotely located monitor. (These units, and their installation are also covered in detail in our book Caravan & Motorhome Electrics)

Typical energy monitors. Pix: Victron Energy, Xantrex.

Typical energy monitors. Pix: Victron Energy, Xantrex.

Most dc-dc alternator chargers and many solar regulators have energy monitoring inbuilt. They are programmable for battery type, size and voltage. Stand-alone units are also available. Consult the battery supplier for LiFePO4 monitoring.

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.

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