Dual battery systems; making sense of the options

[e9999]

at the simplest level, you'd want to have the individually-specced charging upper voltages of each battery be the same or close if you want to charge in parallel. Beyond that, there may be more subtle issues with charging patterns etc, but not as critical. For charging, capacity, current ratings, etc would not matter much -if at all-, AFAIK.

 

[ntsqd]

At the top level the battery chemistry is the important part. Ideally they're the same battery with sequential serial numbers, but even when they're different capacities, state of health, all of that, the charger is the bully, the batteries will fall inline. Just do not leave them connected to each other when not charging.

 

[e9999]

It is very true that the chemistry matters a lot, but that is reflected in the desired charging voltage, the charger does not care about nor knows anything else. But how to go about it would depend on how you propose to charge them (together, one from the other, etc).

 

[kirvesmies]

at the simplest level, you'd want to have the individually-specced charging upper voltages of each battery be the same or close if you want to charge in parallel. Beyond that, there may be more subtle issues with charging patterns etc, but not as critical. For charging, capacity, current ratings, etc would not matter much -if at all-, AFAIK.

@e9999 , ok, consider me stupid like stump. am i understanding correct re: spec'ed charging max volt , meaning if one's spec is 13v, the other one's spec shouldn't be say 14.8v?
my plan was to connect them parallel, for charging n have a total disconnect between engine room n house for running the heater. house would not be tied into vehicle system at all as i'd build a completely independent harness house to heater.. except for pos to pos, neg to neg

At the top level the battery chemistry is the important part. Ideally they're the same battery with sequential serial numbers, but even when they're different capacities, state of health, all of that, the charger is the bully, the batteries will fall inline. Just do not leave them connected to each other when not charging.

@ntsqd , so to see if i'm getting this right, i'd disconnect house from engine room when i get to camp, go about my weekend running my heater. then hook back up to to charge when heading home or charging in the interim? would/should i disconnect just for a fuel stop or say a dinner on the road? like i said to e9999, stupid like stump

thanks for your time n input fellas

 

[e9999]

Every battery type has an ideal voltage for charging. So if you charge them in parallel (not necessarily the best way), if they are different, one (or both) will not see its preferred max charging voltage, because in parallel they will always see (basically) the same voltage. Having said that, in real life the difference is not huge, for instance for "12V" batteries, the ideal max charging voltage is about 14.4V for FLA batteries, whereas for AGM and LiFeP it's more like 14.6V. So, you could set your charger to 14.4V max and all types would be reasonably happy and safe, at least in the short term. Don't do 14.6V if you have an FLA in there, though, that's pushing it and you may boil it.

An issue with disconnecting and then reconnecting the 2 batteries would be if one is much less charged than the other. There would be a voltage difference and the higher voltage one would send current to the lower one until they both reach the same voltage. Nothing inherently wrong with that, but you'd want the wiring to be big enough to handle it.

 

[ntsqd]

That high charge connected to a low charge current flow is what happens when they are left connected w/o some form of charger being operative. From personal experience what happens is that any even tiny resistance in the connection (ground side counts!) turns into a little electric heater and left alone long enough this will kill both batteries, dead. BT, DT.

Look into "Automatic Charge Relay" or "Voltage Sensitive Relay" for automatic separation of the batteries when not being charged. I only buy Blue Sea Systems or BEP Marine products for this use. I'd be willing to buy Victron if they have such a product. I would never, ever buy Sure Power anything.

Short term if you're charging one battery that wants a 14.6V charge at 14.4V nothing will happen. However, the battery will never reach a completely full State of Charge ("SoC" or "SOC"), but it will work. Long term this shortens the battery's life-span.

 

[NookShneer]

I’m approaching a similar system . I’m working to add a Lithium 100ah battery and inverter to a system that is already on a 100 series.

Currently:

AGM start battery - 200w solar and charge controller. When parked battery is always charged to float state voltage 13.8v fridge is wired in and always on to help keep drinks cold.

150amp alternator and Alt-S diode to bump charge voltage up slightly.

Adding:

Lithium house battery 100ah
2000w inverter

Victron Cytrix Li Isolator 120amp (13.3v batteries disconnected, 13.4 combined)

Basic diagram:

I get that a DC - DC charger would be a better choice but trying to maximize gains to the existing system with minimal cost (already have the solar charger controller system in place and always rolling off the system.)

I understand that with the AGM profile on the solar charge controller it wont completely charge the Lithium to 100% but I’ll be in the 90-95% easy and I don’t have an issue with that or lowering the lifespan of the battery as I’m going cheap on this test before a premium battery is purchased.

Other than the size consideration for the isolator/fuse/cable sizes.

Any other considerations while planning this out?

Thanks!

Dave