LiFePO4: what makes a Smart charger Smart?

So what does a "Smart" LiFePO4 charger do?

How would one know the charger they're buying is actually "Smart"?

How much risk to charge the string with a Smart charger but without a BMS and then manually balance?

Or would it be better to: partially charge then manually balance, partially charge then manually balance, repeating until fully charged?

thanks,

A "Smart Charger" is just a charger with multiple profiles and some safety features - by multiple profiles, I mean it may deliver 10A into a fully empty battery, 8A into a 1/2 full, etc...

Their is either a special IC or controller chip or an actual MicroController running the show

All LiFePo4 packs should have built in BMS (if you bought a premade pack) otherwise then balancing can become an issue.

If you could provide these answers I may be of more assistance:

1.) What is the voltage of your pack?
2.) Where did you get the pack (manufacturer or vendor)
3.) Chemistry ? (headaways for instance self balance I believe)
4.) What is the voltage of your "smart charger"
5.) What is the output CURRENT or charger Amps?

-Mike

I'm building my own pack of Headway 38120 cells. 12S for 38.4Vn 10ah with 43.8Vmax.
I've ordered a BMS (43.8Vmax, 24.0 Vmin, 20A Cmax, 30A continuous max). I don't need the higher currents right now, but got them to allow for future use of dumping regen back into the pack and situations where I need a fast charge.

I don't have a charger yet. I'd like to have something in the 5A to 8A range for when I need a fast charge (charging of a generator - get-er done), and have another charger in the 2A to 3A range for general use.

I have a meter to monitor pack voltage, but not cell voltage. So to run it without the BMS until it comes, I figure I need to stay away from the pack Vmax Vmin to keep the cells from being damaged.

My max continuous load is 16 amps, but I don't expect to get above 10 amps until I have the BMS installed.

Using a pair of Cellog 8M before I have the BMS would seem prudent, but if I stayed with a pack voltage range of 26V to 42V, I'm guessing it should be safe.

Can one run Cellog 8M monitors at the same time as a BMS charges, balances? Or are they just a "while running load" item?

Canoe said:

So what does a "Smart" LiFePO4 charger do?

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There are different degrees of "smart" chargers available. Most bulk chargers (not talking about hobby LiPo chargers) sold as "intelligent" that I own will follow the proper charging profile for the battery chemistry they were designed for and show a "green" fully charged state when the current reaches a predetermined setpoint (on most of mine that's around 0.2A). The output voltage on most of these chargers however will stay high and don't shut off. I do have one "smart" charger that'll look at the pack voltage then ramp up the voltage and current a little slower than most. At the end of the charge cycle this charger will also drop its output voltage a few volts (I don't like the feature). It also supposedly shuts off after a period of time, I forget how long, though I have yet to test that. An example of a really "dumb" charger on the other hand, like my cheap 2A/6A, 6/12V auto battery charger, is simply a big transformer with a rectifier circuit. When I connect that charger the amps can soar much higher than the 6A rating depending on the battery's state of charge then the current will steadily drop. If I leave that charger unattended in the "6A" mode the voltage will rise quite high and cook the car battery.

-R

thanks

Russell said:

... At the end of the charge cycle this charger will also drop its output voltage a few volts (I don't like the feature)...
-R

Click to expand...

sounds like the behavior of the Soneil SLA chargers

So, for a LiFePO4 ~36V pack (38.40V) charged up to 43.8V, then until current tapers down to 20%-30% CC, when the charging is complete and the battery pack is disconnected from the charger, what is the measured voltage across the battery pack?

Canoe said:

I have a meter to monitor pack voltage, but not cell voltage. So to run it without the BMS until it comes, I figure I need to stay away from the pack Vmax Vmin to keep the cells from being damaged.

My max continuous load is 16 amps, but I don't expect to get above 10 amps until I have the BMS installed.

Using a pair of Cellog 8M before I have the BMS would seem prudent, but if I stayed with a pack voltage range of 26V to 42V, I'm guessing it should be safe.

Click to expand...

If you aren't going to use a BMS then be sure to charge each cell with a single-cell charger to get each to the same state of charge. Once the cells are charged to the same level you can bulk charge the pack but use a lower overall voltage than you would with a BMS. I don't run a permanent BMS on my 15-cell LiFePO4 pack so I use a 54.6V (3.64V/cell) 2A charger for every day and once in a while I'll hook up the BMS and use a 55.5V (3.70V/cell) 4A charger to top-balance the pack.

After removing the BMS from my pack I checked individual cell voltages after a ride and after charging for about 10 cycles to be sure everything was staying in balance but haven't bothered since (90 more cyles) . As I said though I do balance the pack now and then. The cellog monitors would make checking much easier and...safer too (I was always careful not to cross probes!).

-R

Canoe said:

thanks

Russell said:

... At the end of the charge cycle this charger will also drop its output voltage a few volts (I don't like the feature)...
-R

Click to expand...

sounds like the behavior of the Soneil SLA chargers

So, for a LiFePO4 ~36V pack (38.40V) charged up to 43.8V, then until current tapers down to 20%-30% CC, when the charging is complete and the battery pack is disconnected from the charger, what is the measured voltage across the battery pack?

Click to expand...

SLA's have a charging profile different from Li-ion so all proper SLA chargers should go high then decrease over time to a float voltage, this is normal. The "smart" LiFePO4 charger I'm talking about drops to effectively stop further charging, it's not meant as a float voltage. I do use the charger for every day charging but when I connect the BMS to balance the pack I use the other LiFePO4 charger which stays high at the end of the cycle. When the current reaches 3-4mA, as indicated by an inIine wattmeter, I shut it off and leave the BMS connected for 15 minutes then remove it too.

-R

Canoe said:

So, for a LiFePO4 ~36V pack (38.40V) charged up to 43.8V, then until current tapers down to 20%-30% CC, when the charging is complete and the battery pack is disconnected from the charger, what is the measured voltage across the battery pack?

Click to expand...

Or, a single Headway cell, fully charged at 3.65V, once removed from the charger will measure what voltage across the cell?

Canoe said:

Or, a single Headway cell, fully charged at 3.65V, once removed from the charger will measure what voltage across the cell?

Click to expand...

I can tell you what my LiFePO4 cells are and that'll pretty much hold true for other LiFePO4 cells as well. My 15-cell pack has been sitting for 9 days (I'm injured) and I measured it just now at 50.98V (3.40V/cell). I applied the charger and the battery accepted 0.039Ah/2.0Wh with a voltage of 54.4V (3.63V/cell). That means the battery was 99.6% full at 51V (3.40V/cell) when compared to 54.4V. A resting voltage of about 3.20V is just about fully drained.

-R

I use a Turnigy Accucel 8150 that has 3 different Li settings - LiPo, LiLo and LiFe. It also has NiCd, NiMh and lead settings. In the LiFe mode it has a fast charge, regular charge, discharge and balance setting. Also it will turn off after a programmable period of time or number of milliamps. The charge rate (amps) is programmable as well. It will charge and balance up to 8 Li cells. It lists for $42.44 + s&h from Hobby King - jd

What makes a Smart Charger smart is an algorithym like the one for LiFeP04 attached here. It must be programmed into the the firmware of the charger to be effective View attachment 40138F1 Charge Profile.pdf

Thanks for all the replies.
Learning lots; see there's more to go.

Forgive me, as before this I haven't looked at charging batteries since nicads came out, but don't we need to supply a voltage higher than the cell voltage in order for it to charge? I'm confused about what is happening during the Constant Voltage stage. If we bulk charge Constant Current up to the cell's Vmax of 3.65VDC, then continue with Constant Voltage charging at 3.65VDC, once we're down to 2% to 5% of CC, the LiFePO4 is considered full? Then, removed from the charger, shouldn't the voltage of the cell be somewhat less than the voltage we were charging at(3.65VDC)?
Yet, in the various discharge graphs I find posted around Endless-Sphere, it appears that the cell actually starts delivering current at 3.65VDC, although it quickly drops under load. Do LiFePO4 cells actually remain at the final charge voltage of 3.65VDC? What's happening here?

Thanks,

My LiFePO4 pouch cells are essentially full at ~3.40V with anything over that representing a "surface charge". Once charged to 3.65V and assuming no load of any kind that surface charge will slowly bleed off over time. For example I charged my 15 cell LiFePO4 battery 10 days ago to 54.4V (avg=3.63V/cell) then it sat for 9 days with no load on it, not even a BMS. After 9 days I checked the voltage and it was at 50.98V (avg=3.40V/cell). I charged the battery back up to 54.4V and it accepted a scant 0.039Ah/2.0Wh of power or 0.4% of the 10Ah rating. You can see from these numbers how little capacity there is in the surface charge.

So why charge the cells to 3.65V or higher if they are full at 3.40V? Mainly I think this is done to be sure the cell(s) get fully charged especially when the they are charged in series. Since the BMS will protect the cells against over-voltage and the shunts kick in at 3.60 to 3.70V there's little risk in using a higher charging voltage for LiFePO4. A higher charge voltage also speeds up charging and helps to get a series string balanced quicker.

Note that other lithium chemistries are different in that you charge them to 4.20V and that's pretty much that. You can choose to charge a bit higher to really stuff them full or go a bit lower for longer life but only I'm talkin' 0.1V max.

-R

what chargers would you guys recommend that are smart? what about not smart? would it really matter alot if youre plugging in thru a BMS?

would this charger be able to charge an ebikekit lifepo4 with bms? looks similar to the smaller charger they are offering in the site but at much cheaper
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=250685827921

thedarlington said:

would this charger be able to charge an ebikekit lifepo4 with bms? looks similar to the smaller charger they are offering in the site but at much cheaper
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=250685827921

Click to expand...

I'm not seeing any technical detail about the charger. Only that it's nominally 36V. No info on current or charging voltages. Likely meant for SLA.

nevermind, it only does 220vAC but this one ships from california and can do 120vAC, what do you guys think? it does mention smart charger, and for li-ion battery, will it work for a lifepo4?

Hi Canoe,

I am fairly new to all this but this is what I have done. 2 X 8s 12ah headway packs in series for a total of 16s(only split them for mounting purposes on bike) No BMS. I bulk charge with a soneil 48v. 3.8amps cc up to 3.6v then holds cells at 3.6 cv and tapers the current to zero. Once this happens I connect my single cell(voltfreaks) array and balance charge the cells to 3.65. I am still waiting for my cell logs to arrive for monitoring and alarm port cutting off of the bulk charger. I am doing this manually till the logs arrive. I've only done about 30 cycles so far and the cells stay balanced throughout discharge and during charging within .01v each. From the time the bulk charger finishes @ 3.6 to the time the cells are all balanced @ 3.65 only takes about 3-5 minutes. So far I am happy with this arrangement. The only drawback is the price tag for these chargers. Soneil 48v $200, individual volt freaks $10 each, I needed 16 of them. But then again no one said this bobby was an inexpensive one plus I am impressed by the overall performance of the cells on the bike powering a 5304 crystalyte.

Mike

alright so i ginally bought a cheapie razor scooter charger, im guessing SLA? to sort of get my feet wet and for use with an ebikekit 9ah lifepo4, it just says equal charging voltage 42.6, floating charge 41.6v its got a max of 1.6 amps, i know its slow but i figured it would be something safe to try out, i see that lifepo4 chargers decrease charge until they reach 0v when battery's full, im guessing this one wont? so i just want to ask is there any reason i shouldnt use this charger to charge my kit? its just temporary while i wait for the outcome of GGoodrum and fetcher's boards to start shiping