How would I build a Lithium Ion battery charger?

[src]

Trying to save my laptop which won't charge a battery... I need to externally charge the 14.8 V 3600 mAH battery

Anyone know of any similar projects on the web? Any advice?

Thanks,

Mike

 

[IaPuP]

I've designed chargeers for NiCd and NiMH but the Lion battery chargers are VERY complex. You can charge a battery with just a resistive load and an incoming current, but you need to watch the maximum rates of your cells as well as the maximum charge. If you overcharge a Lithium Ion battery, it WILL explode. If you beat up a Lithium battery, it can explode too. I saw one do it once, when a guy had strapped a Lithium Ion battery pack in as a substitute to his NiCd pack on a radio controlled airplane and crashed it. He threw the thing in his trunk and got ready to drive home and noticed smoke pouring out of the back. He opened the trunk in time to see the battery pack had ripped the plane apart and started the trunk on fire.

I built a delta-peak NiMH charger (that senses charge and turns itself off!!!) with a dozen JFET transistors and a tuned network of resistors, etc. As for LION chargers. I don't think that delta-peak methods work and I'm honestly not sure what methods would be best to use.

If you're brave, you can rig up a voltage-matched input current at roughly 1/20 of the total hourly storage capacity (an 1800mah battery charging at 90mA). It will take roughly 20 hours to charge, but it will give you more leeway so that you don't blow it up if you accidentally leave it charging for a few minutes beyond maximum capacity. With LION batteries, watch the temperature, since they tend to heat up rapidly as they approach maximum charge.

Personally, i've worked with stuff like it and I'd still opt to buy an intelligent charger off eBay...

Eric

 

[gsellis]

I am with IaPuP. Buy one (a smart charger). Even if you do not overcharge it to explosive results, overcharging a LION will shorten its life considerably by accounts I have seen regarding camcorder batteries.

 

[Mark R]

Do not attempt to charge a Li-ion with anything except with a charger controlled either by a specially designed Li-ion charge controller chip, or a microprocessor, connected, at the very minimum, to the temperature sensor inside the pack.

Most li-ion battery packs will contain a protection circuit, that will disconnect the battery internally in the event of overcharge, over-discharge, short-circuit and excessive charge speed. They are, however, designed to stop the battery from exploding and won't necessarily prevent it from being destroyed by inappropriate charging.

Severe overcharge will cause a fire or explosion. Mild overcharge will decimate the batteries life. Over-discharge will destroy a battery in hours.

Most laptop batteries are microprocessor controlled - the internal CPU tells the laptop how much charge is left, how 'young' the battery is, and tells the charger exactly what amount of charge to deliver. Unless you can get hold of the detailed specs of the battery pack (what internal monitoring is included, and what its specs are, recommended charge limits (constant current charge rate, constant voltage switch-over point and charge termination current) and details of the temperature sensor) then you are unlikely to build a satisfactory charger.

 

[FrankSchwab]

Oh, you guys get real.

Li-Ion charging is essentially a current-limited, constant voltage circuit. It can (almost) safely be done with an LM317 and a few resistors.

Caveats:
1. You are not allowed to fast charge LiIon batteries if they are very dead. You Must trickle charge them until they reach a minimum voltage level (3v?)
2. There are temperature limitations on charging (i.e. no fast charging in freezing conditions, no charging at all if the battery is hot).
This description does not take these into account.

Procedure:
1. Create a strictly regulated 4.1V source. An LM317 and a couple of resistors will do this.
2. Calculate the size of a current-limiting resistor. Assume a max current of 1 amp, a max voltage differential of (4.1V - 3.0V) = 1.1V. Resistor size = 1.1 ohm.
3. Hook the battery up to the 4.1V source through the current limiting resistor, taking Caveat 1 into account.
4. Unhook the battery when the charging current drops below 10 ma. (voltage across the resistor = 0.01V).
5. You have a 98% charged LiIon cell.

This charging algorithm is nearly precisely that recommended by LiIon manufacturers, though they make it a bit more complicated - they specify a constant-current source until the 4.1V level is reached, then a constant 4.1V, tapering current charge until the 10ma level is reached.

Note that some LiIon cells are specified at 4.15 or 4.2V for the voltage limit. Play it safe and use 4.1 - the difference in capacity is negligible.

Please play with this in a safe place, with eye protection. Getting this wrong can have disastrous effects.

/frank

 

[Kostya17]

Originally posted by: src
Trying to save my laptop which won't charge a battery... I need to externally charge the 14.8 V 3600 mAH battery
Anyone know of any similar projects on the web? Any advice?
Mike

Sounds like you'll need one of these Since your battery is 14.8V, I assume it's 4-cell, so you'll have to buy a 4-cell unit. Another thing you'll need is a 12V automotive turn signal bulb (less than a buck at Big Lots/Wally). Now, disconnect your lappy's power adapter from outlet/computer and connect it according to the last circuit on the linked PDF file (page 4, lower right-hand corner). The lamp will act as a current-limiting resistor. Plug your power adapter to the outlet--the bulb should light-up. It will dim as battery charges-up and will turn OFF when safety circuit determis the battery is charged. That's it!

Note1: before you buy any parts, find a friend/co-worker/etc. with similar laptop and ask them to charge your battery. It is quite possible your battery is dead, so the your laptop obviously cannot charge it. Using extra charger will not help in this case:-(
Note2: most of the laptops use "smart battery", i.e. it has some additional terminals for communicating its status to host. The original "smart battery"spec requires that battery accepts charging as long as it is not fully charged regardless of the signal status. This was done so the battery keeps itself charged whenever laptop is plugged-in, even if OS is crashed/halted and no signals are passed from host to battery (the HP batteries I used for RC plane work that way). I found out "hard way" that Toshiba laptop battery blatantly disregards this requirement. You will not be able to charge it with such a simple circuit.

 

[src]

Thanks for the replies guys!
Kostya17 your post seemed promising, but I do indeed have a Toshiba battery guess I shouldn't bother?

 

[Kostya17]

Originally posted by: src
Thanks for the replies guys!
Kostya17 your post seemed promising, but I do indeed have a Toshiba battery guess I shouldn't bother?

Please check your battery 1st in your friend's laptop as I mentioned in my post. It's quite possible your lappy is OK and you just need a new battery.

 

[src]

Don't have convenient access to a new battery (i'd have to order one from the net) the laptop works fine and shows how much battery life is left... you think it still could be a bad battery problem?

 

[NeoPTLD]

Don't even bother reinventing wheels.

To charge a lithium ion battery, you start off with constant current, then once certain voltage is reached, you start the second phase of charging at constant voltage. Charging is terminated when the current in constant voltage mode drops to a pre-determined value. The circuit also needs to have dangerous condition shut off.

I have no idea what the actual parameters are, but it's no easy task to build it from scratch. What you want is a battery-charger controller IC(most likely Maxim makes Li-Ion charger controller) and build around it.

 

[Mark R]

If you want a good charger, then yes, you absolutely do need a dedicated charge controller, with temperature monitoring, etc.

If you want to charge slowly (3-4 hours to 90%) then FrankSchwab's circuit will be fine. I'd probably aim for a charge voltage of 3.9V per cell rather than 4.1. 3.9 is often specified as full charge, with the internal battery protection activating and disconnecting the pack at 4.1-4.2V. Note that you should add a time limit or some form of fail-safe cut-off to it.

 

[FrankSchwab]

Actually, 4.30V is where the protection circuitry kicks in, see:
Panasonic LiIon Information
(See the LiIon Charging Information PDF at the bottom)

Most of the Panasonic batteries are specified at 4.2V final voltage, though a few are 4.15V.

/frank

 

[pm]

A li-poly/li-ion charger is a piece of cake to build. Constant-current followed by constant voltage. There are several different designs that I have seen but the simplest is 2 transistors, 3 resistors (and a good power supply) schematic for this is here along with a lot of commentary.

Then there's Scott H's which is here and has 2 active components and 5 passive. There is an entire thread with testimonials, modifications and commentary here on an electric R/C group.

I made the former and it worked fine but I have several 2, 3 and 4 cell packs and I got tired of a fixed rate, fixed pack size per design. So, I picked up a Electrifly Triton for about $115 which does NiCd, NiMH, Li-ion, Li-poly, and lead-acid. If you don't need so many bells and whistles you can get a 2-3 cell charger for less than $50. Sometimes there are ones on Ebay.

If you need a good power supply, there are a lot of good surplus ones on E-Bay. I picked up a very nice 12V 15A power supply off of Ebay for less than $25 including shipping. This can be a cheaper alternative than buying a car battery. Alternatively, an old computer power supply works. Since you have a 4-cell pack, you probably will need a higher voltage supply.

For all the fact that the charger is a fairly easy design, and despite the fact that I am now charging with a very solid charger, I charge my li-poly packs in a small Weber BBQ with the lid on. I have blown up one - not sure why... I believe that one cell in the 3 cell pack, became unbalanced.

 

[Kostya17]

Originally posted by: src
Don't have convenient access to a new battery (i'd have to order one from the net) the laptop works fine and shows how much battery life is left... you think it still could be a bad battery problem?

Yes, it could be. If you're willing to take a risky experiment, then you'll need:
1) Voltmeter
2) Ammeter
3) Current limiter (12V automotive lamp will do the trick)

Assemble the circuit I was referring to earlier with the following changes:
1) No safety guard
2) + from battery goes to Ammeter
3) - from battery goes to - on your laptop p/s
4) Voltmeter should be connected directly to the battery

After you turn on the circuit, 2 things could happen:
A) Ammeter shows some current--circuit works! Watch for voltage--it should not exceed 16.8V
B) No current--that's the problem with Toshiba battery I was talking about :-( Double-check wiring and if it does not help, well, that's it :-(

If (A) is the case, DO NOT LEAVE BATTERY ANATTENDED, IT CAN CATCH ON FIRE IF OVERCHARGED. Monitor voltage and stop the charging when you have 16.8V on the battery. You will need to buy the safety guard circuit if you want to charge the battery unattended.

Edit: Charging a Li-Ion battery without Li-Ion charger... Isn't that "Highly Technical"