LiFePO4 charging?

[jman231994]

I found a charger that can pump out 18A at 36v, so wont have to do two chargers. $88 but I need to make sure it can be shipped here in time for the race in a few weeks time.

It will be school property and it will be rolled over to the next group, however we are currently the first group to have gone in the competition, last year was our first year and this year were going for it again one last time....

The school is happy to support us to an extent but a really expensive charger just isnt a priority right now for the team with money from our sponsors being quite tight so far......Just means we need more sponsors lol

The charger: http://www.bmsbattery.com/alloy-shel...v-charger.html

[SLATYE]

I'm not sure, but that one doesn't appear to have any balancing functionality. It'd be worth having another (slower) charger on hand for that.

Also, it seems to come in 110V and 230V versions. Since it's from a US company, I'm betting that the one they're selling is the 110V model.

[mtma]

I have a feeling that the intent is for the unit to work with one of their BMS units which do the cell level charge bleedoff.

Though the specs are scant so it might be included, who knows - better find out before dipping your toes and ruining your batts though.

[jman231994]

Yep will certainly look into it. Otherwise we can just slow charge it every now and then off my friends rc charger....will just take a while, but it will be balanced

[B.D.R]

Slayte, dunno mate, never balance charged at 20amps, might give it a go later on today and see what happens

I only balance charge after every race day, or with new packs and that is done at 1c, when i want them charged quickly it gets the 20amp treatment, but that is 2s saddles and 3s sticks.

OP, i would'nt use that charger, but that is just me, the amount of wrongness that can happen when a LiPo\LiFe battery goes bad is not worth saving a few dollars on a charger, and remember a LiFe battery is still a LiPo just a bit more stable

What sort of low current protection are you using in this thing?, becaue a LiFe battery can be damaged form low voltage

Originally Posted by jman231994

Otherwise we can just slow charge it every now and then off my friends rc charger....will just take a while, but it will be balanced

No it wont be, there is a reason there are balance ports on LiPo\Life chargers mate, and it aint because the manufactures like wasting money, if the packs you are using are cheapys then correct balancing is even MORE important.

[desertstalker]

Originally Posted by B.D.R

Slayte, dunno mate, never balance charged at 20amps, might give it a go later on today and see what happens

Most chargers do the balancing at the end (during the CV stage of charging) It adds about 5mins to the charge (assuming you have decent packs that are already mostly balanced) regardless of the rate selected for the CC portion.

The larger iChargers are AFAIK limited to 500mA for balancing. The PL8 can do 1A IIRC.

[mtma]

I don't think that's quite true, balancing happens continuously but they let the CC run as hard as it's programmed to.

So in the case that the CC portion didn't allow for enough balancing during the time it took for the CC charge to take place, the cell that has reached maximum voltage determines the remaining rate at which the whole pack can be charged, as the excess current must be bled off on the first series cell that has gone CV - which is limited by the bleedoff current capability of the charger.

[jman231994]

Originally Posted by B.D.R

Slayte, dunno mate, never balance charged at 20amps, might give it a go later on today and see what happens

I only balance charge after every race day, or with new packs and that is done at 1c, when i want them charged quickly it gets the 20amp treatment, but that is 2s saddles and 3s sticks.

OP, i would'nt use that charger, but that is just me, the amount of wrongness that can happen when a LiPo\LiFe battery goes bad is not worth saving a few dollars on a charger, and remember a LiFe battery is still a LiPo just a bit more stable

What sort of low current protection are you using in this thing?, becaue a LiFe battery can be damaged form low voltage



No it wont be, there is a reason there are balance ports on LiPo\Life chargers mate, and it aint because the manufactures like wasting money, if the packs you are using are cheapys then correct balancing is even MORE important.

Ok...that makes sense. I was wondering how it could balance without knowing each cell individually. The cells arent cheapies in electric vehicle terms, they are about mid range. They are designed for electric vehicles in general, with many diy car conversions using them

http://www.evassemble.com/index.php?...products_id=18

We will be using low voltage alarms to warn us when they are getting flat. Although I'd rather not use cut outs because if it came down to dying minutes and we could win by doing just a tiny bit more, I'd honestly rather kill the batteries. It's our last time in the event and we have always had to source our own parts and get sponsorship so I see nothing wrong with making the next group have to do just as much work.

I think we will just go with one of the balancing boards for the entire battery to make life easier and then take it off for the race. Although in the grand scheme of things, the money is needed elsewhere on the bike.

[daztay]

http://www.emtb.com.au/
I use one from this site on my 48V 10A lifepo4 cells they have a 36V charger aswell .
If you use a charger as above youll need a BMS installed to keep cells balanced VERY IMPORTANT .
I have a imax charger with balancer that i balance up my cells every 10 charges. Can only do 6 cells at a time which is why i use the 48v charger.

http://www.hyena-industries.com/
hyena is a member here too.

another forum you might find interesting. It all about electric powered stuff.

http://www.endless-sphere.com/forums/

[jman231994]

Sorry for taking a while to respond. In the middle of exams.

I've been reading into the possibilities of using a standard sla charger for lifepo4 and from what i've read it should work assuming we use bms chips to keep it balanced and if we use a charger for higher voltage because the lifepo4 voltage is higher than the sla equivalent.

What i was thinking is this. We have a really powerful 24v charger at school for our old bike, so if we were to charge half the battery at a time with a bms on each half, would we be able to use that charger?

Kind of running out of time and need to make a decision in the next 2 days so we can decide which batteries to order, so yea just need a brief answer rather than a full technical answer. We could also get some resistors for the charger to drop it to the correct lifepo4 charging voltage

[daztay]

With current charging Lifepo4, the charger needs to shut off when 3.65V per cell is reached. When ppl refer to lifepo4 packs eg 36V it really means 12 cells @ 3.2V which is 38.4V but they need to charge up to 3.65 a cell which would be 43.8V.
Have a read through the endless sphere forum i linked ealier, it has a vast amount of knowledge on the subject. Using a SLA charger could kill the batteries if they are charged greater than 3.65 a cell.

[jman231994]

Originally Posted by daztay

With current charging Lifepo4, the charger needs to shut off when 3.65V per cell is reached. When ppl refer to lifepo4 packs eg 36V it really means 12 cells @ 3.2V which is 38.4V but they need to charge up to 3.65 a cell which would be 43.8V.
Have a read through the endless sphere forum i linked ealier, it has a vast amount of knowledge on the subject. Using a SLA charger could kill the batteries if they are charged greater than 3.65 a cell.

Yep, i've been on endless sphere in the past for info. I could swear I made an account, but everytime I got to make a thread and login it doesnt work. Probably just my memory lol. Ill have a look again when I get a chance

Based on what you said, wouldnt I be correct in saying that I could charge 6 cells at a time off a 24v charger, assuming the bms is capable of preventing an overcharge. Or does it need to be fed with a voltage equal to its full charge voltage.

[daztay]

You could charge with a 24V charger if you bms can handle the 2.1V difference.
Its also possible to use a series resistor, but that is crude.
http://www.hobbyking.com/hobbyking/s...idProduct=5548
I have one of these can charge 6 @ 5A it also has balancing and storage charging.

[desertstalker]

Originally Posted by daztay

You could charge with a 24V charger if you bms can handle the 2.1V difference.
Its also possible to use a series resistor, but that is crude.
http://www.hobbyking.com/hobbyking/s...idProduct=5548
I have one of these can charge 6 @ 5A it also has balancing and storage charging.

Erm, that is quite capable of charging LiFe itself. Even without balance taps to the cells. No need for messing about with chunky resistors.

[Reaper]

Originally Posted by SLATYE

Can it actually balance the packs while doing that?

My understanding was that the little balance connectors are normally limited to something like 2A, so you can't really charge at too much more than that (maybe 10A maximum) before it becomes impossible to balance the cells properly.

Yes. Lipo Chargers charge in two stages. Constant Current and then they switch to Constant Voltage. (CC and CV). The first stage is where the battery is charged. The send stage is where the balancing happens.

This is why it doesn't rank linear. You can save time in a high C rate charge, but the balance phase still takes time, so that is why you do not get a 2c charge rate as exactly twice as fast as a 1c rate.