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mixed Pb + Lifepo

Posted: Jan 09 2022 5:04am
by Tyro65
I know. I know that mixing these into one pack is a bad idea. I do read warnings about it, but I have not found a good explanation. I do want to know the details of what would happen during charge and discharge cycles if I added a 12V SLA to a pack made up of 16 LiFePO4 15Ah cylindrical cells.

Thanks.

Re: mixed Pb + Lifepo

Posted: Jan 09 2022 5:57am
by john61ct
So confused.

What is the xPyS layout, the voltage of your existing pack?

Yes mixing chemistries is bad, for many many reasons but which ones specifically will depend on exactly what it is you are thinking of doing.

You really need a higher level of understanding before you think about breaking standard rules

Re: mixed Pb + Lifepo

Posted: Jan 09 2022 6:22am
by Tyro65
It's that higher level of understanding that I am seeking by posting here.

The LiFePO4 pack is Headway cells 40152S. They charge up to about 55V. I want to add about 13V using a 15Ah SLA. I have a (nominal) 60V charger.

Re: mixed Pb + Lifepo

Posted: Jan 09 2022 6:43am
by TrotterBob
Tyro65 wrote:
Jan 09 2022 6:22am
It's that higher level of understanding that I am seeking by posting here.

The LiFePO4 pack is Headway cells 40152S. They charge up to about 55V. I want to add about 13V using a 15Ah SLA. I have a (nominal) 60V charger.
As previously stated, bad idea. The why is that SLA battery has more internal resistance and a lower C rating than those Headway cells. Better to add a few more 40152s if you can. Even if the Headway cells you already have have been cycled, if they only have a 100 or so cycles they are still practically new since they can last between 2,000 and 4,000 cycles.

Think of it this way, if your pack was all SLA you'd have a much shorter range and reduced performance. So adding in 4 cells (13v) worth of SLA is like having a pack of sled dogs and one of them is a Basset hound while the rest are Huskies well suited for the task. :mrgreen:

Re: mixed Pb + Lifepo

Posted: Jan 09 2022 7:09am
by john61ct
You did not answer my questions. You cannot expect members to write out the thousands of posts required to get your 101 level knowledge up.

Read read read old threads, Google for resources.

But I am inferring you are talking adding the lead 12V in series.

A horrible horrible idea, likely dangerous.

Parallel is just sub-optimal.

Add more Headways, same model but ideally less worn.

Re: mixed Pb + Lifepo

Posted: Jan 09 2022 1:12pm
by Chalo
TrotterBob wrote:
Jan 09 2022 6:43am
So adding in 4 cells (13v) worth of SLA
That would be 6 lead-acid cells. Not that it really matters; it's still bringing the whole pack down to the level of its lamest component.

Re: mixed Pb + Lifepo

Posted: Jan 09 2022 2:36pm
by 999zip999
Don't do it.

Re: mixed Pb + Lifepo

Posted: Jan 09 2022 2:45pm
by Tyro65
Thanks, all.
I will heed the advice. And you've taught me well.

Re: mixed Pb + Lifepo

Posted: Jan 14 2022 7:53am
by leffex
Tyro65 wrote:
Jan 09 2022 5:04am
I know. I know that mixing these into one pack is a bad idea. I do read warnings about it, but I have not found a good explanation. I do want to know the details of what would happen during charge and discharge cycles if I added a 12V SLA to a pack made up of 16 LiFePO4 15Ah cylindrical cells.

Thanks.
Generally no.

Lead acid will add more weight. New lead acid lasts about 100 cycles vs life 3000 or whatever high number they have.

Ah of lead acid are measured by C1/20 meaning if your application dras the lead acid faster you won't get the ah hours stated on the lead battery.

A charger will charge until it reaches the chargers set voltage and will not help you with balancing your pack for you

If you want to use then maybe read, watch more videos and meet local or not local people doing this things already.

If you put in another serie of voltage the average voltage of all series connected will go down and one can get more life of your battery thus using only a lower percentage of full charge.

For your information I used a lifepo4 cell as a security, as a buffer for my 20 series li-ion battery so that when my battery was charging it would eat the voltage at the higher level so that the rest of my battery didn't go above each cells voltage threshold. (lifepo4 charges at 3,75v then goes down to 3,45 after sitting)

I replaced my buffer with 1s65ah lipo for the same reason and after a while of use, I guess, I heard a pop and one of the lipo cells broke and I had to remove it and remember that they aren't as safe as one cell type. There are safe lipo types also but I don't know so better not use it to.

What I did learn. Better use only one type of cell.

Re: mixed Pb + Lifepo

Posted: Jan 14 2022 11:12pm
by john61ct
Not disagreeing with the above but one issue

Top quality lead batteries designed for deep cycling can last 12-14 years in daily use if the system as a whole is designed and cared for properly.

Or 6-8 years with lower cost units like Deka's or Trojan's 6V GCs, not much over $1/Ah at 12V

Yes LFP will last much longer maybe by decades, but since it only takes one "event" to render the whole bank scrap, such a long ROI makes the 8x dollar$ per usable Ah a pretty risky investment.

So going for it should be based on better performance and convenience, not just economics.

But yes just getting consumer grade fraudulently marketed 12V units from big box or automotive channels, 80% SoH at 100 cycles would be normal used for deep cycling, or even less.

Re: mixed Pb + Lifepo

Posted: Jan 14 2022 11:15pm
by john61ct
PS I do not mean used for propulsion at high C-rates

House bank only, solar style storage, getting back to 100% as per datasheet spec at least 4x a week, DoD% averaging maybe 40-60%