This post will introduce you to battery charging and tell you how to charge lithium cells using a bench power supply.
Executive summary:
Attach a single lithium cell to the power supply and set it at 4.20 volts and 0.5C. Leave it charging until the current drops to 1/20C. Repeat for all cells in the pack.
I use lithium cells (LiPo, LiIon, etc.) in a lot of my projects – usually surplus cellphone batteries. They have much better energy/volume and energy/weight ratios than other battery types (the next best is NiMH).
But they have a reputation of being “tricky” to charge, and needing a special charger.
Not really.
Put your money into a bench power supply instead of a lithium charger. The power supply will do the charging just as well, and is tremendously useful for other things as well.
You need a constant voltage/constant current (CV/CC) DC power supply. This lets you set a maximum voltage and a maximum current at the same time.
The Mastech 3003D is typical of inexpensive CV/CC bench power supplies. It goes for $110 to $150 online, and is well worth the money. (Mastech is Chinese, but the quality seems OK. Other brands are fine too.)
Batteries 101
Batteries are made up of cells in series. Each cell has a typical voltage that depends on its chemistry – carbon-zinc and alkaline cells are around 1.5 volts, NiCd and NiMH around 1.2 volts, lithium cells around 3.3 to 3.7 volts. Lead-acid cells are around 2 volts.
To get higher voltages, cells are put in series in a battery. Sometimes there’s only one cell in the battery, as with typical lithium “batteries” at 3.6v or so. (Technically these ought to be called just “cells” not “batteries” since there’s only one cell, but people often call them batteries anyway.)
A cell (or battery) has a capacity rated in amp-hours (AH or mAH). For example, an 800 mAH battery can produce 800 mA for 1 hour, or 400 mA for 2 hours, or 8 mA for 100 hours (at the rated voltage) before the battery is drained. In theory.
In practice, cells are usually rated assuming you’ll be draining them over 20 hours. Most batteries produce more energy if you drain them slowly. So, a 5 AH battery can produce something like 250 mA for 20 hours (0.25 * 20 = 5). If you drained it at a rate of 5 amps, it would die before one hour is up.
Also, the voltage gradually drops as you discharge the battery – it starts a little above the rated voltage, then spends most of the time near the rated voltage. When it starts to drop quickly from there, it’s empty.
The current needed to (theoretically) empty the battery in 1 hour is called C. Charge and discharge currents are given in units of C. For a 800 mAH cellphone battery, C is 800 mA.
Most cells have a maximum discharge current beyond which their performance drops off dramatically. For tiny watch batteries (coin cells), this current is really small – something like 0.002C. (“0.002C” would often be called the “500-hour rate”.) These cells are designed to run a long time at tiny currents. At the opposite extreme are lithium polymer cells designed for high-current uses such as in electric-powered model airplanes. I’ve seen some with a maximum discharge rate of 20C – meaning you can drain them in 1/20 of an hour (3 minutes) reasonably efficiently and without damaging them!
Charging
Most rechargeable battery types can be charged at modest constant currents, without worrying about voltage – something like 0.05 to 0.1C (NiMH, NiCd, lead-acid). So you could charge one of these cell types in 10 or 20 hours with your bench power supply by setting the current to 0.1C or so (500 mA for a 5 AH battery), and just turning the voltage up all the way (so it’s not limited).
(I’m not talking fast charge here – that’s a whole other topic; ask Google about that, not me).
Lithium cells are different. They need to be charged at a constant current until reaching a specific voltage, and then constant voltage. But that’s trivial if you have a CV/CC bench power supply.
Just set the current limit at the initial charge current, and the voltage limit at the ending voltage. When the cell is first connected it won’t reach the voltage limit, so it’ll soak up the full amount of current. As it charges, the voltage will gradually rise until it reaches the voltage limit. Then the power supply will keep the voltage constant, with the current gradually dropping as the cell charges. Once the current flowing into the battery reaches about 1/20C, it’s as full as it’s going to get (no real harm in leaving it longer – the charge current will eventually drop off to nothing, but I wouldn’t leave it there for weeks).
Setting the correct voltage limit when charging is important. If it’s too high, you’ll damage the cell. Too low and you won’t get a full charge. The magic voltage for lithium cells depends on the exact chemistry:
| Cell type | Charge limit | Notes |
|---|---|---|
| LiIon – coke anode | 4.1 volts | Older; rarely seen now |
| LiIon – graphite anode | 4.2 volts | Very common; frequently seen in phones, cameras, computers, etc. |
| Lithium polymer (LiPo) | 4.2 volts | Can take very high discharge rates; popular for R/C cars & airplanes |
| LiFePO4 | 3.6 volts | Mostly used in high-current applications; electric bicycles and cars, etc. |
If you’re using something exotic, check what the manufacturer says. For all the lithium cells I’ve used, 4.20 volts is plenty close enough.
Suppose you want to charge a 800 mAH lithium cellphone battery. Set the voltage limit on your power supply to 4.2 volts. Set the current limit to 0.5C (400 mA). Hook up the cell (positive output of the power supply to the positive side of the battery, and negative to negative). If the cell was empty, the voltage will start around 3.2 volts and gradually rise, with the current at exactly 400 mA. Once it gets to 4.2 volts, the current will gradually drop. When it drops to 40 mA (0.05C), it’s done. This will take 2 or 3 hours, because a 0.5C charge rate means a charge in 2 hours. But once the current starts dropping, the charge gets slower.
Most lithium cells will charge fine at anywhere from 0.2 to 0.8C; never more than 1C. In my experience 0.5C is always OK; slower is a little less wearing on the cell than faster.
Be careful – lithium cells can catch fire or explode if they’re damaged or mischarged. Always keep an eye on them while charging, and stop charging immediately if they swell, start making noise, or get very hot (> 70 C / 160 F or so; a little warmth is normal).
Also, don’t over-discharge lithium cells. Unlike NiMH or NiCd cells, it is not good for them. Discharging below 3.1 volts or so will greatly shorten the life of the cell. Lithium cells are happiest when charged frequently to keep them “topped-up”.
If you do over-discharge, you can try charging it very slowly (no more than 0.1C). Sometimes this brings them back to life (sometimes not). But watch it very carefully while doing so.
Balancing cells
Some lithium cells in a pack will discharge slightly faster than others, especially as the pack ages. One cell can drop to too low a voltage without this being obvious from looking at the total pack voltage. To avoid this problem, it’s best to charge each cell independently instead of the whole pack in series (at least once in a while). This will ensure that each cell is fully charged, “balancing” the voltage in all the cells.
Note that you can charge lithium cells in series; but it’s best to balance them, at least every so often.
Thank you for the info. Killer Page… I found it very informative/useful. Luckily I already have a cc/cv Dual Tracking Laboratory Power supply, but was wondering how to correctly and safely charge my 11.1 20C Li-Po Pack, and your now i’m confident (thanks to you) that I can do so.
The best way to charge your pack is one cell at a time, as described in the post. 20C is the maximum discharge rate for the pack, so that doesn’t tell you the charge current – look at the AH (or mAH) rating and use 50% of that current.
If you want to charge the whole pack at once, you can do it but you won’t be balancing the cells. To charge the whole pack at once, use the same current (50% of the mAH rating), but 3×4.2 = 12.6 volts (because you have 3 cells in the pack).
Good luck!
Thank you so much! I have been looking for such information for quite some time to no avail. This will be very useful in my projects.
Excellent site. Very concise and useful. I’m still unclear about determining current for the cell/bench charger. I have 8×3.6v cells that appear to be 100AH/cell. Does that mean that in series they add up to 800AH? What about in parallel?
Do I need to rewire (series-parallel) the cells for charging and discharging?
Thanks for your help
Peter,
100 AH are huge cells – are these for a electric car? (Or perhaps a Boeing 787? Ha ha – if I’m where Boeing is getting their charging info, that would explain a lot.)
Maybe you mean 100 mAH? That would be a really small cell, for something like a tiny indoor R/C helicopter.
Anyway, no, if the 8 cells are in series then you have a “8S” pack, which is 28.8 volts (3.6 * 8) and still 100 AH.
If you put cells in series the AH rating stays the same, but the voltage goes up. If you put them in parallel, then the voltage stays the same but the AH rating goes up (then you’d have 3.6 volts and 800 AH).
You don’t have to rewire the cells to charge them – if the 8 cells are in series you can charge them at 33.6 volts (8 * 4.2) and around 50 amps (0.5 C). But if you charge them that way you won’t be balancing them – once in a while you should charge each cell individually (4.2 volts at 50 amps) to balance them. Usually people wire packs to bring out the anode and cathode (+ and -) of each cell to a separate “balancing connector” for this purpose.
That said, if you’re really working with 100 AH cells (and not 100 mAH), CONTACT THE MANUFACTURER. Lithium cells of that size are not toys and you can get a serious fire or even explosion if they’re not handled properly (look at the Boeing 787). General advice – like on this blog – isn’t good enough for huge cells like that – you should talk to the manufacturer and do exactly what they say. At a minimum (without knowing any details about your cells), I’d put a thermometer on the cells while charging and stop immediately if they get over 50 degrees C (120 F), and I’d charge them inside a vented, explosion-proof chamber. Far away from myself.
Those precautions are not needed so much for little cells like those used in cell phones or smaller R/C airplanes, but 100 AH cells are not little. (I’ve never personally seen one bigger than 15 AH.)
Maybe what you mean is your cells can DELIVER 100 amps of current. Many small R/C airplane cells can do this – for example if you have a 2 AH (2000 mAH) cell that can discharge at at a 50 C rate, that’ll deliver 100 amps (for a theoretical time of 60/50 = 1.2 minutes; in practice less).
I hope this is helpful. Feel free to reply if you’re still confused. And be careful!
Dave,
Again, very helpful, thanks. I have attached a link to the batteries I am using. I bought a charger from the same site (recommended by them) but I still have some questions about charging and voltage. If nominal voltage is 3.2 shouldn’t I be looking to charge them around 25.6 v for 8s configuration?
I am also curious (this is a larger discussion) about the use of a small solar panel to maintain a full charge for this battery. I plan to use it to power an electric outboard motor for my sailboat. I used it last year with great success but had no charge so when it ran out of juice… I was stuck
Thanks again for your help
http://evolveelectrics.com/Sinopoly_100Ah_Lithium_Battery.html
This is the power supply…
http://evolveelectrics.com/Power_Supply.html
to clarify my last post, I used the battery on the boat, not the solar panel. It seems like trying to charge this battery with a solar panel involves a battery management system at least and maybe something more involved…
for fun, a link to the outboard motor which is a pretty impressive piece of machinery.
http://www.torqeedo.com/us/electric-outboards/cruise-t-tiller-controlled-dinghies-motor-boats-sailboats-up-to-4-tons
Hi Peter,
I’m not familiar with the huge automotive type cells you’re using.
The page you pointed to doesn’t mention the chemistry of the cells, which seems a little strange. I strongly suggest you ask the vendor for their recommendations on charging.
From the size and description, I’m guessing they’re LiFePO4 cells – these normally have a “working” voltage around 3.2 to 3.3 volts, and a charge limit voltage of about 3.6. So if that’s what you’ve got, the proper charging voltage for a 8S battery would be 3.6*8 = 28.8 volts (not 25.6).
However given the large size of the cells, I’d definitely talk to the manufacturer and find out what they recommend as the charge limit voltage (also about current).
Re using a solar panel to charge them, that should be fine, but you’ll still need a way to accurately control the voltage (at least) used for charging. If you pick a panel that won’t supply more than 0.5 C of current (or so), then you don’t have to worry about current limiting, but you will definitely need to worry about voltage limiting.
Most “standard 12 volt” solar panels will produce about 17.6 volts open circuit (no load). That’s not enough to charge your 8S battery. If you use 2 in series, that’s 35.2 volts, which is way too much and will damage the cells quickly.
So you need to take the output voltage from the panel(s) and convert it to the 28.8 volts (or whatever the exact number is) needed to charge the 8S pack.
You might look for a “solar charge controller” meant for LiFePO4 cells that will do this (a regular “charge controller” meant for lead-acid batteries is not what you want).
If you can’t find that, you could just get a regular lead-acid charge controller and use the output of that to power a LiFePO4 charger meant to run off a 12v supply. Or, use the output to run a 110 volt AC inverter, and then plug in a bench power supply into that…not terribly efficient, but it should work.
I hope this helps.
Cheers,
–Dave
Im 90% sure they Lithium polymer batteries. (sino-poly is the maker). I got the tenma charger and Evolve tells me to charge them in parallel. That makes for a nuisance: wiring and rewiring the bus bars etc. but at least it’s a plan.
How do I calculate the load/draw of the motor I will be powering and thereby determine the runtime and distance I can expect from a fully charged battery? I will be charging this weekend. Ill let you know how it goes. Thanks
Peter
one more thing… I noticed this morning that the charge on one pack has dropped to 2.38v/ cell. does this mean that the battery will no longer take and hold a full charge?
Hi Dave
I’m very glad I found your web site. I was trying to find a way of charging a LiPo cell without buying a charger. Your explanation and instructions to use a bench power supply is brilliant. So in to the workshop with the instructions and 3 hours later a fully charged PiPo. BTW this cell is normally charged with a solar panel but since it’s winter it can’t.
Thanks a million.
Ray