LifePo4 Vs LiPo batteries - can I use the former to charge the later?

[Deleted member 94047]

Hi,

I've been reading on battery chemistry lately while researching potential portable charging solutions for the Mavic Air. I have read that LiFePO4 batteries charge to 3.6V per cell while LiPo batteries charge to 4.2 per cell. So does this mean that a LifePo4 battery will not be able to fully charge the Mavic Air battery since the voltage differnce will equalize before the later is fully charged? Sorry if this is a stupid question.

 

[Idrach]

Not a stupid question - as a lawyer acquaintance of mine says "it depends".

You are asking the wrong question - what matters here is the total output level.

The rest of this is based on the DJI standard Mavic Air batteries - ymwv for other DJI products or possibly for 3rd party batteries (although I wouldn't use a battery that had a different nominal output voltage than the standard product. Different mAh ratings are a different thing.)

Therefore what it depends on here is the defined output range of the charger pack. If the battery pack is purely parallel and has no output conditioning, then, yes, you won't get much of a charge at all. However, 4 LiFePO4 batteries in series will give you 14.4V which is above the 13.2V maximum charging voltage for the Mavic Air batteries.

There are also clever electronic things that can be done to rework the voltage - the same sort of things that your chargers for most of your electronics do (the old transformer / rectification thing is, well, old!) but can be done to step up as well as step down. So you could have a largely parallel LiFePO4 system that could output from 5V (to charge USB devices) up to say 24V - and would adjust the voltage to ensure it was delivering the maximum charging current.

 

[Deleted member 94047]

Thanks Idrach! That was a great answer!

"what matters here is the total output level " - so if I understand you correctly, the Mavic Air battery will keep drawing current from the the LiFePO4 battery even if the voltage of the individual cells of the Mavic Air battery is lower than that of the LiFePO4?

And if I may ask a second (silly) question, will 12V at 3A be enough to charge the mavic air battery?

Thanks again!

 

[Idrach]

Sorry for the delay.

I don't think you have quite got it. Think of a stack of 1/2" bricks and a stack of 3/4" lego bricks. You can build a stack of 1/2" bricks (your LiFePO4) which is bigger than any relevant stack of 3/4" (LiPo) bricks. You just need 50% more bricks to do it.

Also you can do some things with electronics to change, say, a 12V output in to a 15V output (google "switched mode power supply" if you are interested - and similar to the inverter devices that can give you 110V / 240V AC from your car's 12V DC).

And if I may ask a second (silly) question, will 12V at 3A be enough to charge the mavic air battery?

12V @ 3A, without any trickery, will partially charge your Mavic Air batteries. I'm not enough of an expert on the power characteristics of LiPo and the specific construction / geometry of the Mavic Air battery (as well as what electrickery they have put in there) to determine how much of a charge you would get.

Frankly, I'd buy something designed for the job. Like the Smatree Power Bank - here's a link to Amazon UK, I couldn't find it on a quick Amazon.com search but you may have better luck.

 

[Idrach]

so if I understand you correctly, the Mavic Air battery will keep drawing current from the the LiFePO4 battery even if the voltage of the individual cells of the Mavic Air battery is lower than that of the LiFePO4?

And to answer the specific question asked - only until the charge on the Air battery was sufficient that its output voltage reached 12V. After that it will stop drawing current.

 

[Meta4]

And if I may ask a second (silly) question, will 12V at 3A be enough to charge the mavic air battery?

For some clues, look at the fine print on your Mavic Air charger unit to see what it's putting out.

 

[Deleted member 94047]

Thanks Idrach. If you are curious to know what happened, yes 12V @3A was enough to trigger a charge on the MA battery but only when the MA battery had not been drained to less than 44%. It would charge from 44% to 90% no problem (even if it takes about an hour); but if the MA battery has been drained less than 43%, the LiFePo4 battery can't sustain the charge. You are right, I should probably look into that Smatree solution, it is a bit pricey but I hope it will be worth it.

 

[Kilrah]

What charger are you using? You're using one right, and not just doing the terrible thing of connecting 2 batteries together hoping to charge one with the other??

A charger has a voltage converter and will make the required output for the battery you're charging from whatever the input is as long as it matches the charger's input specs.

 

[Deleted member 94047]

... not just doing the terrible thing of connecting 2 batteries together hoping to charge one with the other??

I was doing exactly that (don't imagine messy wiring though - I was using the below pictured power tank and connector). But, yeah basically one battery connected to another. This experiment is now over. I just coughed up and bought the Smatree portable charging station.

 

[Kilrah]

OK so you were having a regulated 12V supply, the battery type in the pack was completely irrelevant. And no, not suitable at all.