Battery planning

[Ammonite]

I currently have a house bank made up of two 105ah SLA, a 105ah SLA starter battery and an AGM for the bow thruster (I can't recall the capacity but it works well). Everything is set up for 12v. I also have 300watts of solar charging the house bank and a Cyrix battery combiner between the house bank and the AGM. The setup works well although it would be great to have a larger house bank.

With this in mind I'm planning to upgrade the house bank to LifePO4 at some point and when I do this I was intending to split the solar so that 100w goes to the starter battery and 200w to the new house bank, with seperate Mppt controllers, and to move the Cyrix so that the starter battery charges the AGM. I am also planning to install a 50amp Victron DC-DC charger between the starter and house bank suitably derated so that it doesn't fry the 70amp alternator, which I may also upgrade in time.

Does it make sense to split the solar in this way and once the starter and bow thruster batteries are charged how efficiently will the 100w of solar going into the starter battery charge the house bank via the DC-DC charger. I don't have shorepower on my mooring and I use the bow thruster a fair bit getting on and off the mooring, hence the need to charge the AGM via solar before the next trip. Thanks

Edit: I don't think it's relevant to my specific question but for context the three banks are also charged by a three output mains charger and the alternator via a three way splitter. Both of these connections will be removed from the house bank when I fit the LifePO4

 

[PaulRainbow]

It's likely that when the engine battery is charged the solar controller will go into float and you'll lose 1/3rd of you solar yield. So i wouldn't bother. The engine/AGM should be OK without charging for a few weeks, at least, but if you need to keep them fully charged, fit a small solar panel, say 10w with no controller.

 

[Ammonite]

It's likely that when the engine battery is charged the solar controller will go into float and you'll lose 1/3rd of you solar yield. So i wouldn't bother. The engine/AGM should be OK without charging for a few weeks, at least, but if you need to keep them fully charged, fit a small solar panel, say 10w with no controller.

Thanks Paul but I'm not sure I follow. Why would the starter battery go into float when being charged via the 100w panel, if its also charging the LifePO4 via the DC-DC charger and the AGM via the Cyrix, unless the LifePO4 and the AGM are also fully charged, in which case it doesn't matter if the solar is going to waste?

 

[Daverw]

You are adding losses from the solar by going through the starter / dc to dc. We have just converted to lithium and have starter connected to the trickle charge output from our Multiplus, however when sailing would expect this to kept fully charged by alternator anyway.

 

[Ammonite]

I'm not bothered about charging the starter battery as its always fully charged having motored up the harbour but I do want to charge the AGM bow thruster battery having potentially used it a fair bit to get back on the mooring, which doesn't have shore power, hence my original plan to have 100w of solar go into the already charged starter battery, which would then charge the AGM bow thruster via the Cyrix and also charge the LifePO4 via the 50a dc-dc charger, along with the other 200w of solar connected to the house bank. My main concern is the amount of chatter this will cause the Cyrix as the starter battery won't provide much of a buffer so it will potentially keep cutting in and out every 20 seconds (10 seconds of sufficient voltage to close and 10 seconds of insufficient voltage to open). I'm not worried about the losses going through the dc-dc charger as they are negligible in the case of the Victron XS 50a charger at about 1.5%

Another option is to ditch the Cyrix, direct all 300w of solar to the LifePO4 and fit a second smaller dc-dc charger between the LifePO4 and the AGM for the bow thruster so that it's fully charged in time for the next day, which a dedicated 10w panel wouldn't achieve. Does this sound like a better plan?

 

[jakew009]

I'm not bothered about charging the starter battery as its always fully charged having motored up the harbour but I do want to charge the AGM bow thruster battery having potentially used it a fair bit to get back on the mooring, which doesn't have shore power, hence my original plan to have 100w of solar go into the already charged starter battery, which would then charge the AGM bow thruster via the Cyrix and also charge the LifePO4 via the 50a dc-dc charger, along with the other 200w of solar connected to the house bank. My main concern is the amount of chatter this will cause the Cyrix as the starter battery won't provide much of a buffer so it will potentially keep cutting in and out every 20 seconds (10 seconds of sufficient voltage to close and 10 seconds of insufficient voltage to open). I'm not worried about the losses going through the dc-dc charger as they are negligible in the case of the Victron XS 50a charger at about 1.5%

Another option is to ditch the Cyrix, direct all 300w of solar to the LifePO4 and fit a second smaller dc-dc charger between the LifePO4 and the AGM for the bow thruster so that it's fully charged in time for the next day, which a dedicated 10w panel wouldn't achieve. Does this sound like a better plan?

I think your original plan would work fine (I don’t see why the solar charger on the starter battery would go into float if it was being discharged by the DC/DC). There would be some inefficiencies (albeit the new DC/DC charger is pretty dang efficient).

Having said that, I think it would be better to leave the starter battery being charged solely from the alternator, direct all the solar into the lithium battery, and get a small DC/DC to charge the thruster battery as you suggest.

 

[PaulRainbow]

You motor into the harbour, the solar controller will sense the alternator voltage and go into float. The engine battery is, as you say, fully charged, so at what point will the solar controller revert to charging ? Then the Sun goes down.

The 2nd CD-DC charger would be a much better solution.

 

[Ammonite]

Thanks for confirming. I shall go with the second option. It means I don't need to buy a second solar controller which I can put towards the second dc-dc charger

 

[jakew009]

You motor into the harbour, the solar controller will sense the alternator voltage and go into float. The engine battery is, as you say, fully charged, so at what point will the solar controller revert to charging ? Then the Sun goes down.

As soon as battery voltage drops below the rebulk threshold. In reality it would switch back to bulk the minute after the engine was switched off unless both the starter and the thruster battery was already at 100%.

 

[PaulRainbow]

As soon as battery voltage drops below the rebulk threshold. In reality it would switch back to bulk the minute after the engine was switched off unless both the starter and the thruster battery was already at 100%.

Then, if you keep drawing power from the engine battery the DC-DC charger will lock out the input.

If the solar is still charging, there will come a point when either, the solar will go back into float or the battery voltage will be sufficiently elevated to allow the DC-DC charger to resume charging.

Exactly what happens will depend on the exact set points of the solar controller and the DC-DC charger, as well as how much charge is going into the AGM and the Lithium. Having the AGM in parallel with the LA engine battery further complicates and confuses things.