Battery Balancing

[BillyJ]

Hi Guys,

First post here

I've been looking into mulitcell lipo balancing for a while and I'm struggling to find a single IC/PCB solution which I can buy to balance and charge multiple LiPo cells.

Is there some inherent difficulty in designing/building circuits which can stack and balance(passive or active) cells.

Thanks,
Billy

[tszaboo]

Yes. LiFePO, unlike Li-ion has a characteristic, where the voltage changes very little, with the state of charge (SOC). So a battery with 3.15V migth be 10% charged, while a 3.17V is 90% charged. Most LiFePO chargers will only balance the battery when they are discherged, because there the voltage change/SOC is more. So two cells, having 1% difference in SOC might have microvolts of difference in voltage.

[BillyJ]

Thanks for that, but what about LiPo or Li-Ion?

[tszaboo]

Thanks for that, but what about LiPo or Li-Ion?

LiFePO is very similar to LiPO, just voltage is little lower. Li-ion is much easier, you should not have any problem finding balancers for them.

[David Hess]

This application note from TI discusses the problem with LiFePO4 cell balancing mentioned by NANDBlog.

I wonder if it can be overcome by more accurate voltage measurement.

[digsys]

Having spent years designing Li battery packs for solar race cars, I am in complete agreement with NANDBlog. In fact, for the last several months, I am
working on how to balance 4 sets of 44 packs of each 7 cells for our new car. It is VERY tricky. As NANDo says, the bast place to do the balancing is at either
the bottom roll-off curve or the top sharp roll-up curve. You only have 2-3% to play with, but at least you'll be pretty close.
Our method, with a fresh start, is to tie ALL cells in parallel, then run at least 2-3 TOP and BOTTOM balances (using a fooking big variac and bridge :-) ) 100s AMPS,
then make up the pack.
If the pack is assembled, or even if not, the best method I find id to bring up ALL cells in series and as each cell reaches 4.20 (with 4.25V being max for example),
to shunt the charge current from that cell (ie stop it gaining any more energy). Once they are all at say 4.20V, you can be pretty sure that you are within 0.5 - 1.0 %
or better. This is actually way better than bottom balancing, as all sorts of shit happens down there :-)
Once you have a balanced pack, you should always use total E calculations to determine the state of charge. (and limit current accordingly).
A design I'm currently working on, for only a few series packs, has to be balanced on the long FLAT part of the curve, and it is very tough to get right.
I have to use isolated step-up converters to move current between packs, but that's another story :-)
Balancing along the flat part of the curve, even up to 0.5V variation is near impossible, IF you want to be within a few%

[ncoonrod14]

I also have been involved with large solar vehicle battery pack balancing. The most straightforward way is to charge your cells until the first hits overvoltage at which point you load that cell individually as the rest charge etc etc. However as pack size increases this quickly gets messy.

As far as IC solutions, my favorite is the Intersil ISL78610 which can monitor and balance up to 12 cells and be daisy chained to handle more.

Some other options, the LTC3300, DS2726, and the bq77PL900.

A method I've used is to have a power resistor and mosfet on each cell/module which some host can enable to passively balance the pack as needed. Active balancing requires much more consideration.

[IanB]

LiFePO is very similar to LiPO, just voltage is little lower. Li-ion is much easier, you should not have any problem finding balancers for them.

Eh? No. Not at all.

LiPo is lithium ion polymer. These cells generally have exactly the same lithium cobalt oxide chemistry that cylindrical lithium ion cells have, with essentially the same voltage characteristics. The primary difference is that the cells are packaged in a plastic pouch rather than a metal shell. The plastic pouch makes the cells much more vulnerable to mechanical damage, but it also allows them to be lighter and to be made in thin, flat shapes.

LiFePO4 is lithium iron phosphate. This is a different kind of chemistry with different voltage characteristics (typically these cells are charged to a maximum of 3.6 V rather than 4.2 V). Lithium iron phosphate cells or batteries should not be charged with regular lithium ion chargers because of the different voltages needed.

[Seekonk]

Riddle me this. I have a collection of 18650 cells from laptops.  I want to make a battery pack for my CPAP machine, about 150WH.  On another site it was mentioned you could tell the chemistry by the color of the shrink sleeve. I don't know how true this is.  I do have blue, light blue and green ones.  Any problem with mix and match?

[IanB]

Riddle me this. I have a collection of 18650 cells from laptops.  I want to make a battery pack for my CPAP machine, about 150WH.  On another site it was mentioned you could tell the chemistry by the color of the shrink sleeve. I don't know how true this is.  I do have blue, light blue and green ones.  Any problem with mix and match?

The color of the sleeve has nothing to do with the chemistry at all; it may have something to do with the manufacturer's choice of branding, or simply whatever somebody thought looked nice at the time.

There is every problem with mix and match. When batteries are assembled into a battery pack they should be matched for type, size, capacity, date and freshness. Assembling assorted old cells from different sources and unknown history is not a good idea at all.

However, if you disassemble a single battery pack that was previously working well, it would be reasonable to make some of the cells from that set into a new pack (after testing them first for goodness).

Do remember the dangers of lithium ion. A catastrophic failure can be very incendiary.

Why not consider using a battery pack sold for power tools? You can get one complete with all the relevant safety features and a charger designed for it from any regular big box hardware store.

[ncoonrod14]

@Seekonk What IanB said. Additionally, if you arent 110% comfortable with the intimacies of lithium ion batteries, you should not be using them. Just ask NASA what happens when you screw up https://youtu.be/hm18s-NYLZU

If you're working with lithium ions, especially ones you don't know the history of, treat them like a stick of dynamite.

If things go south, there's really nothing you can do. You'll notice in the NASA video that a regular ABC fire extinguisher will do nothing to a lithium fire. A class D extinguisher which is made for these fires won't even stop it, only slow it down. Stick to professional battery solutions whenever possible and never use a custom lithium ion battery pack without some sort of protection from over/under voltage, over current, and over temp protection.

[reagle]

There are plenty of chipsets from TI, LTC,Intersil, Seiko  and others to do that. LiPOs just have slightly different voltage thresholds, but should be otherwise the same. If you can get way with passive balancing, do that. Otherwise it's a lot of fun chasing things when they go boom- I had the "pleasure" of debugging a TI active balancing circuit, that essentially sticks a DC-DC converter per cell and pushes charge up and down. Painful.. On the other hand simple shunt ones are way easier to deal with if you can handle power dissipation

[KL27x]

Riddle me this. I have a collection of 18650 cells from laptops.  I want to make a battery pack for my CPAP machine, about 150WH.  On another site it was mentioned you could tell the chemistry by the color of the shrink sleeve. I don't know how true this is.  I do have blue, light blue and green ones.  Any problem with mix and match?

You can mix and match if you balance each "S" with the others. For instance, if I had 4 cells of X and 4 cells of Y, which have different specs, and I wanted to make a 2S battery, I'd use 2 X and 2Y paralleled for each "S." If you don't know what you have, you would have to do some testing and whatnot. Also, of course, you have to observe max discharge/charge rate, which you will have to guesstimate by ESR.... or just be very conservative.

Of course, since a CPAP machine is used while you're asleep, and to keep you breathing, no less, there is no way to really advocate using old cells, unless you could be sure they are high quality and have not been abused.

[Red Squirrel]

For something like a CPAP I would use a lead acid battery, much more forgiving.  It can sit on the bed side and have a 13.5v power supply hooked up to it at all times and remain stable.  Hydrogen is somewhat an issue but for a single battery, not enough to worry about.

Though on subject of lithium ion, I've been kinda intrigued by the idea of using them in stuff like small solar or backup applications, for charging multiple in series do you have to worry about balancing if you use protected cells?  Idealy you want to charge them up to like 80% and not 100% as it will increase their life time, too. 

[tszaboo]

LiFePO is very similar to LiPO, just voltage is little lower. Li-ion is much easier, you should not have any problem finding balancers for them.

Eh? No. Not at all.

LiPo is lithium ion polymer. These cells generally have exactly the same lithium cobalt oxide chemistry that cylindrical lithium ion cells have, with essentially the same voltage characteristics. The primary difference is that the cells are packaged in a plastic pouch rather than a metal shell. The plastic pouch makes the cells much more vulnerable to mechanical damage, but it also allows them to be lighter and to be made in thin, flat shapes.

LiFePO4 is lithium iron phosphate. This is a different kind of chemistry with different voltage characteristics (typically these cells are charged to a maximum of 3.6 V rather than 4.2 V). Lithium iron phosphate cells or batteries should not be charged with regular lithium ion chargers because of the different voltages needed.

Well apparently, the Lithium Polimer battery -where the polimer is the electrode, not the casing- did not go into production. And the industry decided to use an ambiguous name. I stand corrected.

[Inverted18650]

Li-Ion do a great job balancing themselves as long as they are the same brand, capacity, etc. I've been at it a while for residential solar and started back in the day by collection old laptop cells and building the packs that way. You will have to pull the banks and check them individually once in a while but I've found if you buy new & similar cells and build your pack, they stay well intune without a BMS. If using salvage cells just make sure you build your packs based on there internal resistance and capacity, not just capacity. The internal resistance increases with age so the cells will vary greatly and you should take care to test and match them properly.