NiMH charging for standby requirement  - Page 1
Forum section: electronics / projects

author: Rufus
author_rank: Super Contributor
timestamp: January 10, 2012, 11:58:12 am
content: I need to design a power supply for some monitoring equipment including an NiMH battery pack.The usage pattern will be off for days or weeks, powered from mains for days, occasionally what it monitors needs to be moved which requires it to run from battery for from a few minutes to an hour or so. The battery capacity is enough to run it for a couple of hours.The environment is comfortable for humans so ignoring self heating the temperature range the battery pack will see is limited. The pack doesn't currently have any temperature sensor. The pack cells are specified to take a C/10 charge for a year without damage.Don't really know what to do. A C/10 charge would be fast enough, but, how to cut off and trickle at maybe C/50 continuously? Don't really want to time it and give fully charged cells 15 hours of C/10 after every mains power on. Would a voltage cut off be practical? The pack has excess capacity so cutting off at 80% charge wouldn't really be an issue.Any suggestions?

author: Unknown
author_rank: Unknown
timestamp: January 10, 2012, 12:13:56 pm
content: I would look at the dedicated battery charger ICs by companies like Maxim. They can do constant current charging and -dV termination with time limit. At C/10, you'll be unable to reliably detect the termination, so I would charge them at something like C/2 or C/3. Be sure to pay attention to the recommendations from the battery manufacturer. If they state that C/10 for a year is fine, than a continuous C/10 should also be acceptable.

author: amspire
author_rank: Super Contributor
timestamp: January 10, 2012, 12:45:54 pm
content: I think a 15 hours charge at c/10 is pretty safe regardless of the charge. With NiMH, I wouldn't do it any longer then about 15 hours. Then trickle charge at C/40 or C/50 as you suggested.If the battery has come straight off charge, the NiMH cells will start off at just over 1.4V and fairly quickly drop to around 1.3V. If the cell voltage is over 1.3V, it is probably not discharged much, so a refinement is to only charge for 4 hours at C/10 if the voltage is over 1.3 volts on return of the mains. If it is below 1.3V, the amount of charge is hard to determine as the cell voltage is very dependant on temperature, so give it the full 15 hour charge.An even safer approach is eliminate the trickle charge totally, as the battery manufacturers really don't like trickle charging for NIMH. The strategy recommended by Panasonic is to charge at C/10 for 16 hours, then stop. When the voltage drops below 1.3V, run the C/10 charge for another 16 hours. With this approach, the battery can probably drop to about 80% charge at times while on mains through self discharge, but it will probably last longer.If you are going to use a fast charger, I think the temperature sensing method (ending the charge when the battery temp is 45 degC, or better still a dT/dV of 1 to 2 deg per minute) seems to be more reliable then the deltaV method for NiMH. The trouble with the deltaV method seems to be that not all batteries look the same to the charger, and I have definitely seen deltaV chargers that work better with particular brands. But if you do not need to recharge in 1 hour, the C/10 method seems much simpler. DeltaV worked better in NiCd where the change was more pronounced.Richard.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 10, 2012, 03:41:11 pm
content: If your load is relatively constant, you could just use a microcontroller to estimate charge without measuring the current. As soon as the battery is supplying the load, increment a counter every second or so. Then when charging, set it to full charge current while decrementing the counter at a rate such that about the same charge would be put back by the time the counter goes back to 0. Afterwards, pulse the charge current something like 20ms every second or so to overcome self discharge.

author: Rufus
author_rank: Super Contributor
timestamp: January 11, 2012, 10:45:47 am
content: Thanks for the thoughts all.

author: ToBeFrank
author_rank: Regular Contributor
timestamp: January 12, 2012, 05:49:09 pm
content: If the device can be unplugged for weeks at a time, make sure you size the battery appropriately or get a low self discharge type. Nimh's can self discharge pretty rapidly.

author: reagle
author_rank: Unknown
timestamp: January 15, 2012, 01:54:48 am
content: TI and Linear Tech make some pretty convenient charger chips for NiMH that will handle all that for you