Battery Charging

[Voltz]

I've been trying to find out for ages what the recommended input current for charging a battery is, I think I heard as a rule of the thumb 1/10 of the total capacity e.g. 2000mAh then 200mA in, so basically I want to know two things,

No.1 - How much current in should I use
No.2 - Where can I find the internal resistance of a battery, the data sheets on their sites aren't being very helpful

 

[crutschow]

The charging current depends upon the type of battery and how long you want to take to charge it. You can find that info, and usually the internal battery resistance from the data sheets for the battery. You may have to calculated the internal resistance by looking at the change in output voltage with load.

 

[ikalogic]

as crutschow says, it depends on the battery type.

From my experience, with ni-mh and ni-cd batterie, using a slow C/10 charge current is the best way to get the most out of your battery.

 

[electronicsfreak]

Forgive me if it seems like I am thread-jacking, but I kinda have the same question as you Volts.

What I've read/ heard is that one only needs to charge a NiMH or NiCd pack at a C/10 rate in order to balance the individual cells Learning about Batteries (that is if what you have is made of NiMH or NiCd cells.)

Say In the same case as Volts I can't find the batteries internal resistance. Would it be possible to charge a pack by placing a resistor in parallel in order to limit the current from a power source and assume the rest will go towards the pack? (in my case I have a 12v 500mA power supply and am thinking of putting a 40Ω 5W resistor in parallel with a 9.6v 2000mA pack to charge it at a C/10 rate)

 

[RCinFLA]

The only thing magical about C/10 for NiCad's and NiMH batteries is this is the highest recommended continuous overcharge current. They can be left on continuously at this level without significant degradation. It makes charger very stupid-simple (and inexpensive). Any higher charge rate and the charge has to be terminated when cell has reach full charge.

LiPo-Ion batteries like in cellphone and laptop computers require charge termination. Typically can take c/3 to c/2 but must not be allowed to go over 4.2vdc. They can not be left on continuous 'trickle' charge.

Every battery chemistry has its own unique attributes and weaknesses and charging profile conditions.

One of the new popular chemistries is LiFePO or Lithium-Iron-Phosphate for short. They are being used in Elect. Vehicles and new power tools. They have very high discharge and charging rates but are very touchy to overcharging which becomes an issue with a high voltage series stack of cells. They have a very high recharge efficiency compared to other chemistries.

 

[Voltz]

I've checked the datasheets for lots of batteries but the internal resistance doesn't feature on any of them and what sort of chip would I use for charge termination, I was thinking about how to integrate an op amp

 

[MikeMl]

I've checked the datasheets for lots of batteries but the internal resistance doesn't feature on any of them and what sort of chip would I use for charge termination

One made specifically (or that is "programmable") for a given battery chemistry. There are dozens of different battery chemistrys.

 

[edeca]

We do this thread almost every week. Check any of the following:

https://www.electro-tech-online.com/threads/nimh-battery-cells-query.107657/#post881758
https://www.electro-tech-online.com/custompdfs/2010/06/MAX712-MAX713.pdf (NiCad/NiMH)
https://www.electro-tech-online.com/custompdfs/2010/06/MAX745.pdf (Lithium)

 

[audioguru]

My Energizer Ni-MH cells came with a stupid charger that is simply a timer:
If the cells are dead then it charges them in 6 hours then shuts off.
If the cells are half discharged then it charges them in 3 hours then over-charges them for 3 hours and they get hot.
if the cells are fully charged then it over-charges them for 6 hours and they get very hot.

A parallel resistor is never used to charge a battery. A series resistor can be used to limit the charging current.

 

[tvtech]

Audoguru you are spot on.

The secret to longevity of rechargeable batteries lies in charging them properly. A lot of people don't know this....ie: Consumers.

I often wonder about the self defeating task of getting Consumers to buy products that are supposedly "Eco Friendly" and then supply cheap, horrible chargers that ruin the rechargeable batteries anyway. Really stupid logic if you ask me....

I also have some Energizer Ni-MH batteries that the supplied charger stuffed up. And they were expensive. Not cheap rubbish rechargeables. Used them in my Canon camera. Died after around 20 charges. And were as hot as hell to to touch when charging...even when put on charge when not flat.

 

[audioguru]

Energizer Ni-MH cells are made in Japan for Energizer. Energizer doesn't make them, they buy them maybe from Sanyo.
Energizer 9V Ni-MH batteries are made in Germany for Energizer. Energizer also doesn't make them.

Guess what it says on my Energizer charger? Made in China for Energizer. Ha, ha.

 

[tvtech]

Guess what it says on my Energizer charger? Made in China for Energizer. Ha, ha.

That more or less explains it.