Mixing diffrent capacity LiFePO4 battery's

[cmckinnonr]

I saw Will's video on the 280Ah grade B battery review.

I am thinking about building some for my sailboat. The electrical loads have started overwhelming my lead acid battery's. Instead of adding another 75 pound battery I would like to upgrade technology and save 160 pounds in battery weight. I will also add that I will be using a Belmar MC-614 multi stage alternator regulator.

Most boats have the ability to choose what battery bank to use ie. 1, 2, or both.

One problem I am thinking of is what to do about the engine's starting battery? I would like it to be Lithium, But I don't want to build another 280Ah battery just for starting the engine. I really want one 50-100 Ah battery for the starting and one 280Ah for the house. Maybe 4x 4S 100Ah? one for starting and three for house?

One search found on this forum was mixing Lead and Lithium, Its seems to complicated and forbidden. I liked the idea as a lead acid battery is a bit more reliable and it can't be disconnected by a BMS potentially destroying the alternator.

Thoughts?

Thanks!

 

[snoobler]

LFP is a non-choice for starter batteries. Most can't deliver the needed current due to both the BMS and the cells themselves.

 

[Steve_S]

Not to disagree with Snoobler, but there are SOME LFP that can be used for starter use. BUT and it's a biggy, you would be looking at High Performance EV Grade LFP that is capable of 5C Discharge rates that support Hi C-Rate burst rates. The cell cost for such is VERY HIGH ! and choices are narrow. The cost would be more or less par with an 18650 NMC cell type battery pack. Alternatly other chemistries could do so as well but that brings other complications.

 

[Dzl]

Not to disagree with Snoobler, but there are SOME LFP that can be used for starter use. BUT and it's a biggy, you would be looking at High Performance EV Grade LFP that is capable of 5C Discharge rates that support Hi C-Rate burst rates. The cell cost for such is VERY HIGH ! and choices are narrow. The cost would be more or less par with an 18650 NMC cell type battery pack. Alternatly other chemistries could do so as well but that brings other complications.

And on top of this you add complexity and complications to the vehicle charging system. Lead Acid starting batteries are pretty good at their job, LFP (in my eyes) is an expensive way to complicate that task. I was interested/curious about this for a while (LTO actually, but similar situation) and I came away with the impression that its a problem that doesn't need fixing.

OP, there is nothing wrong with keeping your lead acid starting battery and using LFP for the house bank. Its not complicated and its very very standard for RVs and boats with LFP house banks. You'll need a DC-DC charger (Victron, Sterling, Redarc, many others) or external regulator (Balmar), but you need one of those regardless to charge LFP from the alternator

 

[newbostonconst]

I have had some very good long lasting Motorcycle batteries. Lithium is better for long term storage vehicles because of the low self discharge. Lead acid will ruin itself in one year if being just stored.

But I don't know of a car battery that works for lithium....I do have 2 Battle Born Batteries and they would not start a car because the BMS shuts them down at any high current rate. They shutdown just powering up an inverter let alone starting a car ( a custom circuit to slowly charge the inverter capacitors has to be added for BB's to work ).

 

[Steve_S]

Here is ONE company of a few that have Lithium Starter Batteries.
BE SEATED BEFORE CHECKING LINK and reassure your wallet that it is safe.
* These are "Starter Batteries" and NOT intended for constant draw usage, and so reflect that.

AUTOMOTIVE LITHIUM BATTERIES – Antigravity Batteries

 

[cmckinnonr]

I have thought about all the comments in this post and thank you all for the input.

It sounds like the Lead starting battery with a DC-DC charging LPO battery's is a good option. This may allow the exclusion of the Balmar alternator regulator due to the limited current for charging. Also the Renogy DC-DC chargers has a current limiter input that could be used with a NC 80 C Thermostat switch attached to the alternator to lower the draw on a near overheating alternator.

But, If a LPO starting battery were to be used. The one question I have is, What were to happen if a 20% SOC LPO were to be connected to a 80-100% SOC battery? In this scenario I would be drawing from house battery until almost discharged, Then to start the engine would switch to both or just the starting battery to start the engine. In my mind the difference in voltage and voltage sag resistance of LPO would cause a lot of current to be transferred to lower SOC battery.

As for a High draw starting battery, an LTO battery can supply a 10C @50%SOC for 10 seconds. That may be enough to get a starter moving.
It sounds insane but the LTO in 6S would put the nominal voltage at 14.4v (80% SOC (+/-)) and 12.9v (10% SOC (+/-)). So at LPO's 100% SOC 13.6v - 14.4v. The LTO battery should sit between 50% - 80% SOC. At least the risk of over charging the LTO is non existent, But over discharge is high. The 40Ah LTO cells clam Over cutoff is 2.9v and under cutoff is 1.5v . LTO seems to have a wide range of tolerance in 6S 17.4v - 9V, Of course this is absolute minimum and maximum. Daly has a 400A 6S BMS!!

The starter on my application is 12v 1.4kw, I'm sure peak draw my be double that.

It always works on paper..
Just a thought.

 

[newbostonconst]

Back 20 years ago when I was a mechanic I frequently saw starters draw over 400 amps.

You could do capacitors and charge them through a resister from the LifePO4 pack....