Very simple charger - unlimited cells

[retread]

Pardon my ignorance, but I was wondering if an electronics geek could please point out why this would not work for LiFePO4 batteries? The charge would be pulsed, but would that have any negative effect on the longevity (or positive)? Found it quite some time ago, but have never tried...

 

[fechter]

That one would work good for NiCd or Nimh. Nice to see what the current vs. capacitance factor is. That's a type of "bad boy" charger.

The problem with LiFePO4 is you need to monitor the voltages of each cell during charge. The output voltage of that circuit could be very high without load, so most BMS circuits would fry. If the BMS could tie into a triac or solid state relay to kill the input, it might work.. I don't think the ripple would be a problem as long as the average current stays in the safe range for the battery.

That circuit is also quite dangerous since the batteries would be directly connected to the line. Touch it and you die sort of thing. An isolation transformer would make it much safer, but if you have a transformer, you might as well use it to step the voltage down.

 

[retread]

Thanks for the great answer fechter.

I was thinking of making a charger - more out of curiosity than necessity (and not for my pack) - using that circuit sans any sort of BMS. The idea being that pulsed input would be provided to each cell independently, and the charger would "fly through" the pack using... ahem... mechanical switching relays (it is a bad boy, right?). Each cell would receive, say, 10 seconds at 10 A, then the relays would switch to the next cell (a123s). A timeout would stop the whole process, and would be set such that the amps flowing would be minimal at that point.

All the balancing would thus be done on the charge side of things, with the controller handling low voltage cutoff, probably set a touch higher than normal.

Do you, or anyone else here, think that this idea would work? Dangers aside, would it be able to charge as many cells at an arbitrary amperage as there are relays?

Thanks for exercising your neurons! Maybe this could be as cheap and dirty a solution as possible - that works.

 

[Link]

I was thinking of making a charger - more out of curiosity than necessity (and not for my pack) - using that circuit sans any sort of BMS. The idea being that pulsed input would be provided to each cell independently, and the charger would "fly through" the pack using... ahem... mechanical switching relays.

Great minds, eh? I just ordered one of these relay boards to go along with a pair of Voltphreaks individual cell chargers. 132 relays. Each one is actually two DPDT 30V 2A DC relays sharing a single five volt coil. I can't believe how perfect they are for what I need them to do. I plan on charging them up most of the way with a common SLA charger, and then having some of these relays move a pair of individual cell chargers down the line to top off two cells at a time.

$20 shipped from Apex Electronics.

 

[fechter]

If you're going to use relays to multiplex the charger, then you should have some sort of voltage regulator so the cells never go higher than 3.7v or so. In this case, you might as well get a switching power supply with a good output current. I'd be worried that at some point the cell voltages would get too high and you could fry a cell.

The bad boy in the diagram would be great for a high voltage pack.

The BMS circuit that Gary and I are working on might be usable with that kind of input if the FET was upgraded to a (much) higher voltage or possibly replaced with a triac or solid state relay on the input.

 

[monster]

i tried that circuit a while back. it worked at low amperage but the capacitors got hot and it was inefficient. (i had a few blow too) just like using a light bulb really.

 

[fechter]

It might work better if you used a motor starter capacitor from an induction motor. Those are made to take some current without overheating.