Charge Voltage for NiMH - Page 1
Forum section: electronics / beginners

author: EEBlackSheep
author_rank: Contributor
timestamp: March 05, 2016, 06:26:13 pm
content: I am having a "discussion" with a motorcycle manufacturer about problems with their (new model) motorcycle not keeping batteries charged when running. We are constantly having to take off the battery and charge it with a charger specific to NiMH.So these bikes have a very minor external power requirement. They are an EFO bike with a manual kick. The battery is only required to power the ECU and the fuel pump for 20 seconds while its kicked over and started. Once started, the battery is not required.Anyways, this bike is not charging the battery while running and we have to pull them out and charge them externally.The batteries are Energizer NiMH and I presume they have 7 cells.The battery voltage at rest, room temperature, unconnected is 9.10VThe battery voltage when on the running bike, with engine RPM at 3000RPM is 9.05VWe have tried several batteries and get the same result.I dont know a heck of a lot about NiMH, but will 9.05V charge a 7 cell battery? Is a higher voltage needed?thanks

author: retrolefty
author_rank: Super Contributor
timestamp: March 05, 2016, 06:58:30 pm
content: No, not to full charge anyway. First you have to be absolutely sure of the number of cells being used as otherwise we would just be guessing. I wonder why you don't see the voltage reading higher once running on the generator/alternator? Can't fully charge any battery if the charging source voltage is equal or less then the nominal battery voltage?

author: IanB
author_rank: Super Contributor
timestamp: March 05, 2016, 07:34:35 pm
content: When you say "presume" 7 cells, can you count them?Seven cells is an odd number. Six cells would provide a nominal 6 x 1.2 = 7.2 V battery.NiMH cells are properly charged with a constant current supply. A "float" charge would apply a current of 0.1C, or 1/10 of the battery capacity in amps.It is not good or feasible to use a constant voltage float charge with NiMH batteries. Unless you balance the voltage very carefully, the batteries will consume too much current, overheat and be destroyed.For example, a six cell battery, fully charged, at rest, at room temperature would have a voltage somewhere in the range of 8.1 to 8.7 V. A Seven cell battery would be more like 9.4 to 10.2 V. If trying to use a constant voltage for charge, the voltage should be kept somewhat lower than these numbers to avoid risk of overcharging.From the numbers you give, your battery does seem to fit in the 7 cell range, which would be 8.4 V discharged, about 10 V charged. Giving it a charge voltage of 9.05 V, or 1.3 V per cell, would be reasonable in that case. The voltage is low enough to keep the battery partly charged, and not so high as to overcharge and damage the battery. But as I said this is a bit of a balancing act. It is not a robust design.I could not say exactly what the problem is with the system, other then perhaps the batteries themselves are of bad quality. Energizer has been notorious in the past for selling crap NiMH cells. A picture of the battery and an indication of the cell size would be interesting. Are they AA size cells?

author: EEBlackSheep
author_rank: Contributor
timestamp: March 05, 2016, 07:47:24 pm
content: These batteries are 9V form factor. I can only guess as to the number of cells because Energizer does not list them on their specs.Here is a picture:

author: EEBlackSheep
author_rank: Contributor
timestamp: March 05, 2016, 08:03:51 pm
content: Looking at the package, I see some five print that says 7.2V pile. That would be 6 cells at 1.2V per cell. In that case 9V is enough to charge it.Hmmm, doesn't explain why we are going through batteries. I will have to fire it up and see if current is being put back into the battery when the bike is running.

author: IanB
author_rank: Super Contributor
timestamp: March 05, 2016, 08:10:55 pm
content: [Edit: I just saw your latest post.]Oh, it's a standard consumer size 9 V battery?It has six cells inside it. You can tell by the description where it says it's a 7.2 V battery. You have 7.2 / 1.2 = 6. If this battery is being charged while the motorcycle is running it must not have more than about 8 V across its terminals. Any more than that on a continuous basis and the battery is certainly being destroyed. These 9 V batteries are notoriously difficult to charge reliably. Trickle charging is about the only safe option for long term reliability.One thing you might try instead is to replace the battery with a standard 9 V alkaline battery. Wrap it in plastic in case it leaks, but I suspect it might work fine for quite some time. Since it will be about 9.5 V when new, it will simply act as a backup battery and get stopped from discharging once the motorcycle is running.

author: IanB
author_rank: Super Contributor
timestamp: March 05, 2016, 08:17:02 pm
content: Quote from: EEBlackSheep on March 05, 2016, 08:03:51 pmActually, 9 V is far to much for a trickle charge and will destroy the battery in short order.

author: Simon
author_rank: Unknown
timestamp: March 05, 2016, 08:49:52 pm
content: Sounds like something is wrong with the charging system on the bike.

author: biffster
author_rank: Contributor
timestamp: March 05, 2016, 09:06:27 pm
content: It's normally the current that you care about when charging a NiMH battery rather than the voltage specifically - you just need whatever voltage is needed to drive the current you need.I've built a solar NiMH battery charger before using a charger IC and it's difficult to charge them properly unless they're pretty discharged when you start. The IC that I used detected the battery as being fully charged with change of temperature, change of voltage and also had a timeout function so one of these things could be stopping charging too early if they're not set up correctly. Maybe measure the current to the battery to see if any current is being driven into the battery? If not then you probably have a problem with the charging circuit.

author: IanB
author_rank: Super Contributor
timestamp: March 05, 2016, 09:47:52 pm
content: Quote from: biffster on March 05, 2016, 09:06:27 pmThis is true, but it is difficult to use "smart" charging methods on a 9 V battery because of the six cells in series. If the cells are not equally balanced, or one of them is slightly weaker than the others, then the battery will slowly degrade and stop working properly. It is much more satisfactory to charge 9 V cells slowly using a constant current.For example, with the 150 mAh battery pictured you would use a charge current of about 15 mA and let it run for 12 to 16 hours or so (or in this case for as long as the motorcycle is running).To test this, put a current meter in series with the battery and see what the charging current is. If the charging current is much higher than this it is probably too much.

author: EEBlackSheep
author_rank: Contributor
timestamp: March 05, 2016, 09:59:59 pm
content: Thanks all for the info. I will measure the charge current and get back to you.

author: Audioguru
author_rank: Super Contributor
timestamp: March 05, 2016, 11:35:14 pm
content: Your batteries might be failing because they are old. I bought my Energizer 9V Ni-MH battery 2 or 3 years ago. It is marked 8.4V 175mAh and has 7 cells. Your old battery has 6 cells and less capacity.Both the new one and your obsolete one are shown on Energizer's US website (click on Tech Info at the bottom of the page.

author: EEBlackSheep
author_rank: Contributor
timestamp: March 06, 2016, 12:26:29 am
content: The 3 batteries are all less than a month old. BUT these might not be the correct battery - I will check with the manufacturer.BTW they all hold a charge fine, its just that they get drained as the bike is used, and do not get re-charged.I will check all the batteries and also check the charge current and report back.

author: IanB
author_rank: Super Contributor
timestamp: March 06, 2016, 12:34:41 am
content: Quote from: EEBlackSheep on March 06, 2016, 12:26:29 amAre they meant to get recharged?