8 cells.. 9 chargers.

[Ypedal]

One of those "I"m sure I do this ? .. But.." type questions.

Lets say i have an 8 cell 12ah LiFePo4 pack.

Pack has 8 cells with each a + and - ballancing 16 gauge wire with andersons( seen below )

If i plug 8 x Voltphreak 2 amp chargers, one per cell.. all 8 red lights come on and 2 amp charge per cell takes place.

If i take a Soneil 24v 3 amp SLA charger, adjust the onboard pot for 28.5v ( 3.5v per cell ).

Q: Can all these chargers be plugged in at the same time ? Is this a bad idea for some reason i'm not considering ? other than it's 9 chargers for a 24v pack..

I'm looking to shorten a 10ah drain from 5 hrs charge ( 2 amp chargers ), into a 2 hr charge.

 

[Randomly]

it should work.

 

[flip_normal]

Damn, thats a cunning plan!

How about two chargers and get it down to an hour?

 

[ping]

It will work. But finally, when all the cells are fully charged, the voltage of the battery will be higher than your 28.5v charger. Your cells and Voltphreak chargers will charge your SLA charger

Also, in theory, cells could be over-charged by your 28.5v charger if there's no BMS. But I don't think it will happen.

 

[Randomly]

yes it is possible to overcharge a cell without a BMS if the pack is highly unbalanced, but if you've used the per cell chargers previously they will be fairly close. If your pack is reasonably well matched it should work fine.

Also having the bulk charger set at 28.5V, well below the end of charge pack voltage helps a great deal. The lower bulk charger voltage gives you a good amount of charging headroom in the cells to avoid a cell overcharge from the bulk charger and letting the per cell chargers finish the job.

Although the voltphreaks charger string will eventually exceed the output voltage setting of the bulk charger, the current drain of the bulk charger in this instance should be minimal. It would be something to check but it's unlikely to have any significant effect. In the worst case it just clamps the voltage somewhere above 28.5V, but that will harm neither the chargers nor the batteries. I believe I brought this topology up some time ago in the PIC based BMS thread.

 

[tostino]

Hmm, when the battery voltage exceeds the charger and starts to feed back, couldn't you just fix that by placing a diode on the charger line? That will stop any extra current from possibly harming your controller, and you don't have to worry about taking it off at just the right time.

 

[Ypedal]

Hmm.. that right there is one aspect i had not considered. ( pack voltage higher than SLA charger max set value) :? .. thanks for bringing that up !! 8)

Keeping in mind that this is not a " Plug it in and go to bed " charing solution.. it's more of a " Plug it in and keep an eye on the VP chargers.. at the first green light remove the SLA charger and let the VP's finish the job . then get back on the road..

Could also use one of those on/off 24hr timers with the red and green dial switches, using only a single red "off" tab..

knowing the ah's used ( bike has a CA meter ) set the sla charger for an early shut down.

Go G and R .. get those bms's done !!!! some of us are in a hurry ! hehe..

 

[Randomly]

Hmm, when the battery voltage exceeds the charger and starts to feed back, couldn't you just fix that by placing a diode on the charger line? That will stop any extra current from possibly harming your controller, and you don't have to worry about taking it off at just the right time.

Yes a diode would address that potential problem, but I think it's probably not necessary. It's highly likely that having the output of the bulk charger pulled a few volts above it's regulation set point will cause no problems at all with the bulk charger, nor will it put much of a load on the Volthpreaks stack to drag them down. Just try it and check the current flow into the bulk charger with a meter.