Hi
I've a 24v battery pack made of 20 AA 1600mAh cells. I need to build a slow chrger which is capable of terminating the charge at the right time even if it's connected when the pack is already partially or totally charged, and which do not damage the batteries even if it stays connected for days. It's for a cordless vacuum cleaner. https://forum.allaboutcircuits.com/...ery-management-ic-please.133830/#post-1114367
it should have a 32v DC max 500mA input so I can use the ac/dc power supply I already have. budget 20 euro.
thanks for your help.

 

p.s. the original charger doesn't recharge the battery anymore since when I've replaced the old cells with new ones. I've looked TI website but they don't seem to have any IC capable of recharging a 20 cells pack.

 

:BUMP:

 

Nobody answered because nobody charges a 20-cell pack.

Why do you have a 20-cell NiMH pack? This is doomed to failure. NiMH cells are twitchy. If you over-discharge a cell, or god forbid reverse-bias it, it's dead and that'll screw up your whole pack.

And with such a big pack, it'll be virtually impossible not to deplete and reverse a cell. As you discharge the pack, one cell will be the weakest and will exhaust its capacity long before the others. When this happens its internal resistance rises and the other cells reverse bias it as another load. Then it is DEAD. You'd need a discharge balancer as well as a charge balancer and it would be a nightmare.

Better off using a [few] high-capacity Li-Ion cells and a boost converter. Or just 6 Li-Ion cells. It'll hold 22-23V for a good part of its discharge curve. There are nice balancing chargers and discharge monitors for Li-Ion.

 

Hi InspectorGadget
thanks for the info, it's an electrolux cordless vacuum cleaner (

) which is no longer in production, they keep selling the spare battery packs tough at 70+euro plus shipping, so I tought I would save some money by replacing the cells, I didn't consider that's a "smart" battery so now I find myself with a battery pack that it's not recharged because the charger doesn't recognize the battery (it has a serial communication interface between charger and battery). now since I've already invested some money in the cells I would rather spend few more in a cheap charger rather than binning the entire thing (for sure I will not spend another 85€ on a new battery that lasts 1 year)
So far the only project I found is this http://electronics-diy.com/electronic_schematic.php?id=813 but I don't like it for 2 reasons
1) Shipping expences from australia for the microcontroller would cost me a fortune
2) the firmware is not public, so I'm not going to rely on a firmware written by an unknown hobbist for safety reasons
I would rather build something based on specific battery charger IC from a well known brand, unfortunately all ICs I could find can handle max 16 cells.

 

To build acharger based on the method decribed below could be done using an LM317 as the constant current source and using either a plug-in timer or a CD4060 to limit the charging time. I fell it is less than ideal because, as perviuosly described, it makes no allowance for cell balance but given the circumstances it may be appropriate.
http://www.powerstream.com/NiMH.htm
Overnight Charging
The cheapest way to charge a nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mAH battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours. Examples of this kind of charger are shown at http://www.powerstream.com/Ni-6-200.htm . This charger uses a microprocessor to report the state of charge via an LED as well as performing the timing function.

 

Thanks Albert, that's for sure an easy enough project that I could try.
In the mean time I found this http://www.componentshop.co.uk/500ma-smart-charger-for-12v-24v-race-packs-10-20-cell-pack.html and this https://it.aliexpress.com/item/24V-...lgo_pvid=21caf286-9feb-4534-801d-0748f9037dd1
they seem a bit more advanced and safe since they terminate the charge based on delta V so they can charge also partially charged cells and they have also a temperature sensor for overtemperature or dT termination. I'd like to know how to make something similar...

 

The second one would not be suitable as it is for minimum 3000Ah batteries, but the first one looks like it would do the job and, given its price, it would not be sensible to try to build something to replicate its functions.

 

true I guess I'll end up buying that one, no fun but save a lot of time.