Extremely simple "charge while using" device. - Page 1
Forum section: electronics / beginners

author: corrado33
author_rank: Frequent Contributor
timestamp: November 23, 2015, 06:42:56 pm
content: Question: Would it be safe to leave a NiMH battery connected to a 1.35V source indefinitely? NiMHs can be charged to that 1.35V with no problem. Once the battery reaches 1.35V it'll stop charging, which is fine. It's not fully charged, but I don't care.The application for this is just like a car battery, except on a bike. Someone has a dynamo that has a converter to let it output 5.5V. If he put 4AA NiMHs in series, they'd all get charged to 1.375V, which is still fine. But then he wants to also run a light as the same time.I think as long as he provided some current limiting capacity when the batteries were really low he'd be fine. Sure, the batteries would charge really slowly when they were almost at 1.375V, but that's not really an issue.I may be ignoring the resistances between the battery and charger, but I don't think it'd make that much of a difference. The internal resistance of the batteries should be much higher than the wires, and I'd need some current limiting anyway, which would be much higher than that.Ideas? I'm being a complete noob.

author: Simon
author_rank: Unknown
timestamp: November 23, 2015, 07:55:00 pm
content: what about a gel lead acid battery ?

author: corrado33
author_rank: Frequent Contributor
timestamp: November 23, 2015, 08:05:42 pm
content: Quote from: Simon on November 23, 2015, 07:55:00 pmUnfortunately it's not an option. Whatever battery pack it is has to fit on the handlebars of a pedal bike.

author: Simon
author_rank: Unknown
timestamp: November 23, 2015, 08:09:07 pm
content: You can get some fairly small 6V gel batteries. I am not sure of the charge regime of NiMH, I thought a temperature sensore was used to determine when they came to full charge.

author: Andreas
author_rank: Super Contributor
timestamp: November 23, 2015, 09:14:10 pm
content: Quote from: corrado33 on November 23, 2015, 06:42:56 pmDepends mainly on temperature.At room temperature (20-23 deg C) you could go up to around 1.45V/cell.At higher temperatures you will have to reduce the voltage.At full charge the current should go below C/30 .. C/100 for NiMH.The cells will not last indefinitely. But usually around 3 years.With best regardsAndreas

author: jeffsf
author_rank: Contributor
timestamp: November 25, 2015, 10:59:14 pm
content: NiMH batteries are generally constant-currentcharged and, to the best of my knowledge don't like float charging too well.For example,http://www.powerstream.com/NiMH.htm:Quoteorhttp://batteryuniversity.com/learn/article/charging_nickel_metal_hydrideQuoteI'd definitely look at a battery chemistry that is more amenable to float charging, such as a small SLA battery. I'd definitely stay away from lithium-based chemistry as that is even tweakier than NiMH and can result in fire or explosion.A quick check using one of my on-line sources of parts shows things like a cylindrical, 1.35" dia x 2.41" tall, 2V, 2.5 Ah SLA, so there are definitely some relatively compact options available out there. While that might not be the right battery for your needs, I suspect you can find one that meets your physical restrictions.