mixing sla with lifepo4?? possibility?

[fabieville]

Can I connect a 12v 40ah lifepo4 with a 12v 36ah sla in series to give me around 24v at 36ah with no serious issues develop. I now have a 12v 36ah sla battery and I am planning to buy (4) CALB 3.3 Volt 40AH lifepo4 batteries with connectors and Mini-BMS and connect them in series to get a total of 12v at 40ah then connect them in series with the 12v 40ah sla to give me 24V at somewhere around 36Ah I think to power my 24v 700watt ebike.

Do you think I could power the ebike with this configuration without causing any issues to either batteries. I am planning to disconnect the connection before charging and charge each pack from its own dedicated charger.

I know that sla battery volts sags a lot faster than ifepo4. Will I be able to maintain close to the 24v volt required and at the same time get to use up to the 36ah I need for my extended range? Please tell me the pros and cons of this setup.

 

[dnmun]

no, no, and there are no pros, just cons. lead is dead. 24V is too small, CALB is low C.

why not just use a regular 36V lifeo4 battery?

 

[wesnewell]

To answer your question, it won't hurt to put them in series. I assume you have a 24V ebike system. Just don't go below 21V (10.5V on the sla). Unless you need 12V sections, it will be better to connect all 8 cells in series and get an 8s bms for the final pack.

 

[amberwolf]

Will I be able to maintain close to the 24v volt required and at the same time get to use up to the 36ah I need for my extended range?

If you read around the forums about lead (SLA) you'll find a lot of references to and discussions of it's severe Peukert effect at the loads we place on it, meaning you might get as low as half the Ah (when it's new, less when it's used, a lot less when it's old).

So depending on the load you put on it, you could end up with what amounts to an 18Ah pack, never being able to use even half of your LiFePO4's capacity becuase of the limitations of the SLA.

Other than that, it would probably work, until the SLA runs out of juice and sags under load far enough. You'd have to determine that point experimentally on your actual system, terrain, winds, etc.

 

[dogman dan]

You can do it, but because of the lead section, it will be hard on the lead. So treat it like you have a total battery that is half the size of the lead. Stop when the lead gets low, there will be a lot left in the lifepo4 at that point. If you pull 36 ah from that lead very many times, it will kill it. You can kill lead dead in 30 days that way, in just a few cycles.

Assuming this is just temporary, till you can afford more lifepo4, the longer you wait to complete the lifepo4 battery the worse that eventual lifepo4 will be. So later on you may have two 12v lifepo4 batteries in such different condition that they will also have to be charged separately.

So to sum up, yeah you can do it, but it's a bad idea, only to be done if truly desperate.

Flip side, a smaller 24v lead battery running in parallel with a smaller 24v lifepo4 battery could work better. Still have to charge each one with a different charger, but it could work ok that way.

You'd be much better off to start with as big a 24v lifepo4 as you can afford. Then if you must, add lead for a bit more range. 24v 20 ah lifepo4 might take you farther than you think. Charge before returning if at all possible. 24v 20 ah should cost similar to 12v 40 ah.

 

[dnmun]

no, it is a waste of money. you will end up spending $360 to buy 4 of the CALB and end up with a horribly heavy battery which will not be able to deliver the power needed to make the bike go at a decent speed.

this is even if the old SLA will be able to deliver 20A at all.

so just spend the $360 to buy a 36V20Ah lifepo4 battery pack you can use for years.

 

[dogman dan]

For sure, whether he goes 24v or 36v, whatever you were going to spend on the huge calbs, spend it on a smaller 24 or 36v lithium pack. 24v 20 ah should get you at least 15 miles, and riding very slow, possibly 20. should have about 15 miles range, if you ride about 18 mph, which is not that damn slow.

Then figure out how to use what you have effectively, It might be combining bike and bus, or charging while you work, or whatever can do the trick till you can afford the range you need.