NiMH fixed voltage charge - Page 1
Forum section: electronics / projects

author: Magnets
author_rank: Contributor
timestamp: September 03, 2016, 02:42:22 pm
content: I'm charging 3xAAs (S) using a small solar panel that outputs around 100mA and using a TL431 circuit to fix the voltage at 4.1v max (1.36/cell).It's a simple circuit but essentially provides constant current (ish) then floats the voltage to 4.1v. Is this OK for the cells? They are not really being trickle charged because it terminates on voltage, but they are constantly being charged and not allowed to drop to a resting voltage until it gets dark.

author: nctnico
author_rank: Super Contributor
timestamp: September 03, 2016, 03:53:57 pm
content: No. But there are many chips which can do this job for you. Maybe that will be cheaper & easier than coming up with a circuit yourself.

author: tggzzz
author_rank: Super Contributor
timestamp: September 03, 2016, 06:25:34 pm
content: Simple principle: RTFDS and RTFAN, where DS is data sheet, and AN is application note.For NiMH in general, seehttp://data.energizer.com/PDFs/nickelmetalhydride_appman.pdfFor the maximum allowable trickle charge current, see the data sheet for the specific cells you are using. If you can't find that information, then either get properly specified cells, or cross your fingers and hope.N.B. the max trickle current for NiMH is significantly lower than that for NiCd.

author: amyk
author_rank: Super Contributor
timestamp: September 04, 2016, 01:09:32 am
content: Quote from: Magnets on September 03, 2016, 02:42:22 pmAlthough it won't fully charge it, that would be suitable for a single lion cell...

author: IanB
author_rank: Super Contributor
timestamp: September 04, 2016, 01:19:11 am
content: Quote from: Magnets on September 03, 2016, 02:42:22 pmThis is essentially OK since NiMH cells are fairly robust and can tolerate a reasonable amount of abuse. Good quality NiMH cells can be charged at a 0.1C rate for a long time without damage. So if your cells have a 2000 mAh capacity and you charge them with a constant current not exceeding 100 mA, then this will only be a 0.05C rate and you don't even need to regulate the voltage. Just feed the current through them during the day and let them discharge during the night. They will last for ages. (NiMH cells are special in that they can cope with overcharging by safely dissipating the excess energy. Your TL431 voltage limiting circuit is overkill and unnecessary.)

author: Audioguru
author_rank: Super Contributor
timestamp: September 04, 2016, 03:29:24 am
content: Quote from: IanB on September 04, 2016, 01:19:11 amNo, you should read the Energizer Ni-MH battery manual where they say overcharging reduces the life by causing too much heat and pressure which damages the cell.

author: IanB
author_rank: Super Contributor
timestamp: September 04, 2016, 04:28:23 am
content: Quote from: Audioguru on September 04, 2016, 03:29:24 amThere is a difference between mild overcharging and severe overcharging.

author: D3f1ant
author_rank: Frequent Contributor
timestamp: September 04, 2016, 04:34:53 am
content: From that energiser datasheet: "a maintenance (or trickle) charge rate of less than 0.025 C(C/40) is recommended. The use of very small trickle charges is preferred to reduce the negative effects of overcharging"I've seen other datasheets recommend trickle charges as low as 1/100C and suggest the life expectancy of NiMH batteries in backup power supplies will have a serviceable life of around 5 years.

author: IanB
author_rank: Super Contributor
timestamp: September 04, 2016, 05:08:28 am
content: Right, but that refers to a trickle charge 24/365.When a battery is being charged from a solar cell it will be cycling between charge/discharge every day, so it won't be a constant trickle/maintenance charge in the same way.For example, a standard charge of 0.1C for 16 h will be applying an overcharge condition for the last few hours and this is perfectly fine.

author: D3f1ant
author_rank: Frequent Contributor
timestamp: September 04, 2016, 05:25:33 am
content: Indeed, the chances of over charging from a solar panel are remote if the cells are actually cycled.

author: tggzzz
author_rank: Super Contributor
timestamp: September 04, 2016, 07:41:33 am
content: Quote from: D3f1ant on September 04, 2016, 05:25:33 amWith inappropriate design, the chances are 100%. RTFDS, RTFAN, and follow the information therein.

author: Audioguru
author_rank: Super Contributor
timestamp: September 04, 2016, 01:02:53 pm
content: My Chinese solar garden lights used to have a very cheap Ni-Cad cell that rusted away in a couple of months. For the last few years they use much better Ni-MH cells that do not rust but they fail after about 1 year (almost 365 charge-discharge cycles). I do not think they are overcharged since their 300mAh of rating runs out in about 6 hours of driving the 15mA circuit (only 90mAh is discharged so maybe the charging is only 100mAh). American Energizer Ni-MH battery cells last for years. I think a good Ni-MH cell will last for 1000 cycles.