# NiMH charging and capacity?



## wigginsn (Jan 9, 2008)

Got a Q on charging some new NiMHs.

They are 2100 mAh low loss, made by Matsushima. I’m charging them using an IMax 5 smart charger with delta V, temp and capacity shutdown – although I don’t know how the temp bit works as there is no sensor, but the NiMH charge function has a max temp rise parameter to enter.. I set the charge current at 1.5A or 0.71C based on Panasonic data which recommends between 0.5 and 1C rate.

I’ve done more than 3 full cycles now, and the charger is shutting off at 2100 which is the set capacity. I want to know if I can get more charge in.

Should I: 
[*] Increase the capacity shutoff in the charger (to what?) and just rely on delta V? Could this stuff the batteries? 

[*] Shut down the charger at rated capacity and put up with it? Will this shorten battery life or am I just short changing my run time?
[*] (bonus Q) Stop being so anal retentive and just enjoy running trains?








[/list] 
Cheers
Neil


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## Del Tapparo (Jan 4, 2008)

First I've heard of a charger that had a pre-set shutdown capacity. I guess I would go with letting delta-v shut it down. That's how mine works. Personally, I would lower the charging rate to .1C or .2C.


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## Greg Elmassian (Jan 3, 2008)

Trying to get more than the manufacturer specifies is like trying to get more horsepower by running the motor over redline. 

Hope the analogy hits home... you are taking a risk, and you might not get any more power but a damaged motor. 

Regards, Greg


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## K27_463 (Jan 2, 2008)

First of all, your pack is rated at 2100 mah, and you state the shutoff is at tha tsame 2100 number.That is strange as the charge /capacity equation is not that exact. When charging , expect that at least 10% and more like 15% of measured input during charging is waste heat. So your 2100 shutoff is really less. You are charging at 1.5c, which WILL make heat. A 1.5 c rate is quite high. Ii would try a cycle again with charge rate of 800mah or so, less heat that way and still pretty fast charge, with very little chance of cell damage.

Jonathan


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## Greg Elmassian (Jan 3, 2008)

The true test of capacity is DISCHARGE, not current input as Jonathan says. 

I have a Maha, which will discharge the battery at a reasonable rate and show the milliamps you get. 

I agree 1000% with everything in Jonathan's post and have about 500 rechargeable batteries... (lotsa gadgets and loose cells and stuff other than trains). 

Regards, Greg


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## Michael Glavin (Jan 2, 2009)

Neil,

I'm know expert but have lots of experience with these batteries, specifically evaluating, charging, discharging and load testing for RC Aircraft....

NiMH cell technology with regard to charging is more difficult than all other technoligies. NiMH batteries like their NiCd cousins (Nickel based) are charged by feeding them current at fixed numbers, voltage varies. So we use fixed current settings generally derived from the batteries capacity rating and varying voltage typically up to or around 1.5V per cell and look for a bump in V curve as the battery reaches full charge.

Fast charging (above C/10 is less desirable as charge efficiency is reduced even more) with a temp sensor is best as the minus delta V bump is less pronounced with NiMH and is also temperature dependent at higher currents. NiCD and NiMH batteries both use dT/dt and or -dV/dt to terminate charge; however a proper NiMH charger uses micro-processor controlled -dV/dt algorithms that are specifically written to look at the voltage as the pack charges (essentially pulsing current on/off) temp is ignored. In any event dT/dt is the best charge regimen at C/10 IMO. Problem with -dV/dt is your likely to over charge the cells unless your working with rates below C/10 due to heat building/temp rise.

FWIW:[/b]
While a NiCD charger can be used to charge NiMH batteries its ill advised as battery voltage depression is smaller, and harder to detect than with NiCd batteries. 

As far as limiting the charge to a specific number i.e., “capacity” NiMH technologies require something around 150% of rated capacity input to deliver 100% of rated capacity. In other words charging efficiency is poor. Your charger likely looks at the capacity limit as an additional control parameter, some use time too.

NiMh requires multiple complete charge/discharge cycles to come to life (cycle at rated capacity) so your on the right track. Your charger likely has a temp sensor accessory to utilize its dT/dt algorithm.

I’d suggest you charge your batteries @ C/10 with either charge algorithm this will provide the best case for cell V balance and long life IMO. 

2100mAh = C or capacity
2100mAh/10=210mA
210mA/1000=0.21A

Regards,
Michael


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## wigginsn (Jan 9, 2008)

Thanks guys, valuable input as always.

Looks like I need to drop the charge rate to 0.1C. Jonanthon, can I just check, C = 2100 so 1500mA = 0.7ish. Not sure how you arrived at 1.5C. Did I miss something?

I re-read the charger manual after posting (if all else fails..) The capacity limit is "calculated by a multiple of the charging current and time. If the charging capacity exceeds the limit the process will be terminated automatically when you set the maximum value". 

No mention of the algorithm used or if a heat model based on C is incorporated, but since the "Charged Capacity" indicated for the last two rounds have been 2100 on the button I'm picking thats the termination mechanism. The 2100 value is a number I entered during set up, not an inbuilt thingy. I might try to shut this "feature" off and see what happens.

Greg, I hear you on the Maha and discharge for rating capacity. They were available here but at over $230 I went a bit cheaper. May not have been the best decision in hindsight. 

Cheers
Neil


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