Mixing LiFePo4 and AGM

[Tanner Lynd]

So I've got a little beginner's system I put 3 x 35aH AGMs on in parallel for a total of 105ah. Panels are 200W for now, but not a factor for this question...

I can use a couple of those AGM's on another application, which would leave me one 35aH AGM. I want to upgrade the system to LiFePo4, so if I bought a nice and cheap 100aH LiFePo4 to replace the 2 AGM's, could I wire the LiFePo4 and the one remaining 35aH AGM just to get the addtional amp hours? Would the AGM drag down the LiFePo4 and ruin it?

Let's go one further: If I left the 3 AGMs inline and added the new LiFePo4 so the mix was more balanced would it have bad affects on the LiFePo4? 105ah AGM to 100ah LiFePo4?

 

[eabyrd]

Well now, you will get a variety of opinions on mixing those two chemistries, but I think those against would say your more likely to cook the AGMs while charging the Lipo than hurt the lipo4s. If you search Lithium AGM you’ll find many strings discussing them

i do it, but I do it knowing I leave some of the top end off my LifePO4 and I don’t keep the AGM attached all the time

 

[sunshine_eggo]

So I've got a little beginner's system I put 3 x 35aH AGMs on in parallel for a total of 105ah. Panels are 200W for now, but not a factor for this question...

I can use a couple of those AGM's on another application, which would leave me one 35aH AGM. I want to upgrade the system to LiFePo4, so if I bought a nice and cheap 100aH LiFePo4 to replace the 2 AGM's, could I wire the LiFePo4 and the one remaining 35aH AGM just to get the addtional amp hours?

Yes - mostly.

Would the AGM drag down the LiFePo4 and ruin it?

Only if left paralleled with no charging for a very long period of time.

Let's go one further: If I left the 3 AGMs inline and added the new LiFePo4 so the mix was more balanced would it have bad affects on the LiFePo4? 105ah AGM to 100ah LiFePo4?

Generally, no.

Since LFP has a higher WORKING voltage, it will provide the bulk of its capacity before the AGM start contributing meaningfully.

When sitting, the AGM will pull a small current from the LFP to stay floated.

When you parallel the two, it's like you're adding an AGM backup to your LFP battery. AGM RAPIDLY drops into the high 12.XV range when discharging; LFP will give up most of its capacity BEFORE the AGM start contributing.

 

[Will Prowse]

Just keep in mind that a huge danger of these types of configurations is if an AGM has a shorted cell (which will happen eventually), and your lifepo4 packs feed into that cell (considering their low internal resistance, can push a lot of current), your AGM will create a lot of heat. It will probably cause the over pressure relief valve to open. Battery could melt, or the lid could pop off, worst case. Really messy, really dangerous.

I tell people not to do it. But if you do, use a fuse for each parallel string of packs.

 

[eabyrd]

I tell people not to do it. But if you do, use a fuse for each parallel string of packs.

fair point and sound advice whenever you’re paralleling strings. Thanks Will

 

[Johncfii]

Why not use both banks, but not simultaneously? Wire them through separate circuit breakers, to connect either bank to the system, one or the other, with the appropriate charging protocol set up and selected, for the different chemistries. Then you could also add to the lithium bank later without new problems.

This is what I do. I have an old, large, AGM bank that still stores lots of energy. I keep it charged for reserve power in case of a string of cloudy days. I use the large LiFePO4 bank for daily cycling. My Outback Power Mate3 lets me load the appropriate charging parameters, previously saved to an SD card, in less than one minute.

The AGM batteries have a significant self-discharge rate compared to the Lithium. Either keep them on a separate little trickle charger, or connect them to the system, perhaps once a month, to keep them “topped up”.

 

[Tanner Lynd]

Wow, thanks so much EVERYBODY. What great points all of you have supplied me to ponder! So much great critical thinking to turn the light bulbs on in the brain. The first best point is to migrate out of simple to a more robust and elevated system. Will supplies the best reason to consider safety first, and it appears there is plenty of reason to apply it. (Plus I'm very lazy and don't want to clean up any messes! ) That's exactly why I wanted to ask you guys before I did anything on this.

Awesome feedback guys. I'm very honored to have this place and all of you as a resource. Thanks so much again everybody.