Switching out the six 160Ah AGM’s with Lithium. Why? The AGM’s aren’t holding their charge. They collectively can discharge when new 50% of 160Ah x 6 = 420Ah. That’s 1.2Kw for 4 hours. But as they’re old we get less than half of this, say 210Ah. The charge rate of AGM’s is slow. So most of the 1.6kw solar is either used immediately or wasted, we just can’t bank it for the evening, which is when we will most want it. Conclusion ... bin the AGM’s.

There’s plenty of Lithium formats, brands and qualities. Battleborn seem to be the sailing baseline, but manufactured in US tricky to import into UK. The best alternative UK option (I think) is Roamer. And the forums have plenty of positive feedback. It also sems very conveniently that the single 460Ah battery replaces size for size a single AGM. And frees up lots of weight and space for future expansion. Decision two batteries can suck up and deliver 250A continuous, 500A peak, enough to drive the 2.5kw inverter and any single or collection of devices attached to it.

These batteries can suck up the 133A of solar quickly refilling stores in the evening sun ready for the continuous overnight loads. Result! But...solar isn’t the only source of charge, we also have generator, shore and two engines. From what I understand the inverter/charge siphons any unused AC source energy into the battery store. But the low resistance of lithium puts the engine alternators at risk of burn out at low engine rpm. And then there’s the ‘load drop’ risk of high voltage spike should the BMS cut off the supply (or a manual switch).

There are two options to protect the alternators. One limits current via a battery to battery charger. These are expensive and we have 80A alternators. Victron only have max 30A while Sterling have 70A or 120A options. This approach limits the useful charge of the alternators. It’s a compromise, and requires space for the battery to battery chargers. Option two, controls the alternator based on its temperature. This seems like a more clever, lower risk option. Potentially limiting current may still burn out an alternator in a very hot engine room. So better to control the risk directly and get maximum charge from the alternators if possible. This option still has a risk of voltage drop, which the starter battery should absorb.

I’ve decided to double up the 30 Amp controllers in parallel on one engine to charge the batteries (for now). At some point I might buy additional controllers and connect up the same system on the second engine if needed in the future.

Like the MPPT controllers I’ve built fans into the board with a 40C temperature sensor to keep the units cool.