How to treat NiMH batteries

Rod73

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Can anyone tell me the best way to treat NiMH batteries? To get the maximum life, should they be recharged only when fully discharged or is it OK to top up each day? My new battery lasts me 2 days commute (30 miles) if I put in a reasonable effort myself and do not have strong winds. Should I recharge every 2 days or after each day? I have read conflicting reports about this on the web.
 

bray

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This website has lots of info on NiMh batteries in the pdfs available http://www.panasonic.com/industrial/battery/oem/chem/nicmet/index.html# but it doesn't answer your specific question. My Giant Twist electric bike came with a Panasonic battery and a Panasonic charger especially designed for the job. It's recommended in my bikes manual to "refresh charge" the battery every 15 cycles to maximise battery life. A refresh charge is when the battery is fully discharged (the Panasonic charger has a special circuit to do this) and then fully recharged (according to the manual). The ideal situation for maximum longevity is if the battery is used until it's discharged as much as the bikes circuitry will allow and is then recharged without delay.

In the situation you describe above, recharging every 2 days would give the best life but I doubt there would be much difference if the occasional extra top-up charge after a windy day was thrown in. This would be especially so if you refresh charge regularly (assuming your charger is so equipped - you never mentioned the bike brand/model).
 

Rod73

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Thanks for the info, I will check the links and aim to recharge every 2 days. My bike is a Giant Twist as well and having just invested a silly amount of money in a new 9Ah battery I want it to last as long as possible!
 

bray

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Have you read the latest AtoB about these 9Ah batteries? It appears as if either some 6.5 Ah ones are accidentally going out as 9 Ah or some of them are mislabelled. Does the yellow Panasonic label correctly identify the battery as 9000 mAh? What sort of mileage increase are you getting out of it compared to the 6.5 Ah one? I have a complete 9 Ah battery (with painted panel) waiting for me at the LBS at long last (well at least thats what I ordered)... only problem is I'm to poor to pay for it at the moment so will need to pick it up in a few weeks.

I'd be interested to know in advance if they make a difference to the range because AtoB found no difference with their 9 Ah battery (perhaps they got a 6.5 Ah one instead of the 9 Ah one they were told they got).
 

electric6

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Hi there,

As far as i know you answer is only half correct. Yes, its important to do the refresh(conditioning) charge once every 15/20 cycles.

However recharging after a 50% discharge should prolong battery life, so in this case its better to recharge every day.

Here's a link

http://www.mpoweruk.com/life.htm#dod

To further complicate things don't try to top off the battery pack if you pretty sure its full ( this is good practice with lead acid batteries but not with nimh) It is possible to damage nimh battery packs by continually switching the charger on and off when the pack is already charged.
 

Rod73

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I can confirm that my battery is definitely labelled 9000mAh. It also has at least twice the range (30 miles) of my 2 year old 6500mAh battery. I used to get 30 miles out of my original battery when it was new though.

However, I put a Shimano 8 speed hub into the bike last September, in place of the original 3 speed one. The result of this is that the bike goes faster than it used to before the power cuts out, so the range is reduced! This means that I am not sure whether the capacity is greater than the original battery or not. At the moment I would guess that the battery is about 8000mAh. I will test the bike over the next few days and avoid using it in 8th gear to simulate riding with my old 3 speed hub. That should give me a better idea about the capacity. I shall be annoyed if it turns out I have paid £250 for a 6500mAh battery.

I dropped my old battery about a year ago and cracked the case. Also, the LED's have stopped working so I have no idea how much charge is in the battery or even if it has fully charged, so I needed a new battery anyway.
 

Rod73

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Hi there,

As far as i know you answer is only half correct. Yes, its important to do the refresh(conditioning) charge once every 15/20 cycles.

However recharging after a 50% discharge should prolong battery life, so in this case its better to recharge every day.

Here's a link

http://www.mpoweruk.com/life.htm#dod

To further complicate things don't try to top off the battery pack if you pretty sure its full ( this is good practice with lead acid batteries but not with nimh) It is possible to damage nimh battery packs by continually switching the charger on and off when the pack is already charged.
Fascinating! This is what I mean about contradictory advice, but the information that you referenced does look convincing.

From what I can see from the article, running my battery flat every 15/20 cycles would achieve the same result as going through a refresh cycle. Is this the case?

How can I tell if the battery is full? I have some LEDs on the side but I do not completely trust these. Can I tell by accurately measuring the voltage?
 

bray

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I sent an email to Panasonic about the subject of cycle life vs depth of discharge after reading this and the reply came back including: "A more shallow discharge will yield a greater number of cycles." The person who replied does not, however, have any specific data for this including whether it follows a linear or non- linear graph for their cells. I've since emailed them again to ask for this information to be obtained but in light of the post in conjunction with what I've so far heard from Panasonic, I'll be changing my usage and following that advise, trying not to let the battery discharge much before recharging it.

With regards to whether using the bike until the battery is completely flat gives the same result as a refresh, no it won't. The reason why is that the charger's refresh function is especially designed to discharge the battery at an ideal (relatively low and slow) rate. Using the bike (even on econo) will result in much higher current draws and a very different discharge profile. The bikes own electronics may also cut-out the battery at a different voltage to what the refresh allows it to go down to (although neither allow it to dischage totally). The reply from Panasonic reccomended it never be allowed to fall below 1.0 V per cell and the website pdfs recommend a minimum of 1.2 V/cell where large numbers of cells are connected in series as on the Twist battery.

As long as you've travelled some distance on the bike (so the battery is at least partially discharged), there should be no concerns about recharging it. This is because the chargers that come with Twists are of an intelligent design to prevent over-charging and they'll switch off (in practice the red light flashes) at the right time. According to AtoB magazine, there were some problems with earlier chargers cutting off to soon and Giant arranged for users with such chargers to have them fixed or replaced.

If you've dropped the old battery, who knows what damage may have been done. The manual I have refers owners of damaged batteries to the dealer to get the battery inspected before any further use but if your LEDs no longer work then there's clearly been some level of damage. It's not for me to recommend you take the 2 year old pack apart to inspect it and effect a repair (especially given all the warnings against this on the battery and in manuals due to possible short circuits and fire etc).

The workshop service manual for the Twist lists A271GE0001A9 as the part no for the LFREE TWIST B/P COVER SET (B/P meaning battery pack). I doubt whether Giant UK will sell you just a cover set for the battery at a reasonable price but you have nothing to lose by asking, especially if the housing cracks/LED probs can't be repaired with glue and solder etc On the other hand if the old pack performance is poor, then you may not wish to spend anything on it (even though the cells could eventually be replaced at a much lower cost than a new battery).
 
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