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trougnouf avatar image

Cyrix-Li-ct: will it overcharge/damage the gel/starter battery?

Will the Cyrix-Li-ct battery combiner overcharge/damage the Gel starter battery? The gel and LifePO4 batteries are connected together whenever one of the batteries has >= 13.4V (and the LifePO4's VE.Bus BMS isn't sending a charge disconnect), so when the alternator is on (rare), when the MPPT/solar is working (daily), and when the MultiPlus-II is charging the LifePO4 (likely often). Is this ok for the gel starter battery?

If this is ok, then I don't need to have a separate trickle charger for the gel starter battery right?

battery chargingCyrix Battery Combinerstarter battery
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Mike Dorsett avatar image
Mike Dorsett answered ·

This will depend on the voltage set points for the various chargers, and the max charge voltages for the gel and the Lithium. In general, it's not good practice to parallel different types of battery.

IF: the charger's set points are ALL set to the lowest of the Battery max Charge voltages, then this charging set up is ok.

IF any of the set points are higher than the max charge voltage for a particular battery, that battery will eventually be damaged. Lead acids - even AGM's will tolerate up to 15.5V for short intervals. In AGM's the rapid gassing produced by this voltage forces the gel away from the plates, diminishing the capacity of the battery.

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I guess setting the charger to a gel battery's voltage point would likely defeat the purpose of a LifePO4 specific charger and not be very good for it. Oh well.

So the solution is to have a DC-DC charger ("Orion-Tr Smart DC-DC Charger Non-Isolated") from the starter battery to the LifePO4 battery (to be used when the alternator is on)? Do you know if there is a chance that charging the starter battery (with the MultiPlus-II) would trigger the DC-DC starter to think that the engine is on?

On the bright side, the MultiPlus-II seems to have a separate starter battery chorgur (it's no longer in the specs sheet but mentioned in the manual). Otherwise the solution sounded really messy (DC-DC charger from starter battery to the LifePO4 battery when the alternator is on, and an additional charger to ensure the starter battery is charged)