Why Can't You Use a Split Charge Relay With a Lithium Battery?

The use of a split charge relay or Voltage Sensitive Relay (VSR) with a lithium battery is not recommended - in fact we wouldn't now recommend these older technologies for charging any deep cycle (leisure) battery, but for lithium batteries it's a particularly bad idea.

Lithium Iron Phosphate (LiFePO4) batteries, which are the predominant type of lithium battery for leisure applications, require a precise, carefully-controlled charging process. A split charge relay or VSR is essentially an electro mechanical switch that connects the alternator regulator to the leisure battery, via the vehicle's starter battery, so it will simply pass current through to the leisure battery at whatever voltage the regulator supplies. Now firstly, these regulators put out either a fixed voltage (in older alternators) or a highly variable voltage (in the case of newer 'smart' alternators) and secondly, there is no compensation for voltage drop over the cable carrying current to the leisure battery. Consequently, the voltage that the battery ends up 'seeing' almost certainly won't be what is recommended by the battery manufacturer.

So you can see that in using a split charge relay or VSR that there is no control over the charging voltage at the lithium battery, meaning there is no control over the charging process, which in the best case can reduce battery life and lead to poor charging, and in the worst case could seriously damage the battery, or worse.

Lithium batteries have a much higher energy density than lead-acid batteries, so an uncontrolled charging poses a greater fire risk, and in the worst case scenario, a lithium battery fire is much harder to deal with than a lead-acid one. 

So in summary, using a split charge relay or VSR is simply not a safe or viable option for charging a lithium battery. A DC-DC (battery-to-battery) charger with a dedicated lithium setting that has the proper voltage and current controls, coupled with a reliable Battery Management System (BMS - either built-in to the battery or separate) are critical to maintain the longevity and safety of a lithium power system when charging from your vehicle's alternator.

Our DC-DC chargers are all suitable for use with lithium batteries - click here to view our range.