2 Comments24 July 2024
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 thelithium 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.
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James Garrity
25 July 2024 | 4:43
I'd be interested in why you suggest that you thing a VSR 'wouldn't now recommend these older technologies for charging any deep cycle (leisure) battery'?
Hi James, thank you for getting in touch. The main reason is that a split charge relay will just put out an uncontrolled voltage from an alternator so it will not put the leisure battery into the different charge states it needs to become fully charged so you can never fully charge a leisure battery from a VSR.
David Clarke
25 July 2024 | 19:34
Thinking of a lifePo4 battery in caravan towed by Euro 6 car, can you recommend a charger to wire in
Thank you for your comment - I would recommend using a Sterling Power Wildside Charger for this - https://www.12voltplanet.co.uk/sterling-wildside-caravan-battery-to-battery-charger-12v12v.html
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