Jump to content
Sign in to follow this  
MitchellJames

How long Do Ni-MH Batteries Hold a Charge?

Recommended Posts

How long will a fully charged 7.2v battery stay fully charged for?

If I charge up some batteries and take them on holiday will they still be good to go after 4 or 5 days?

Thanks

Share this post


Link to post
Share on other sites

NiMH batteries start to self-discharge as soon as they are disconnected from the charger. BITD racers used to peak their packs before each race, trying to time it to the last second so that the battery had as little time as possible between being taken off charge and the start of the race.

For bashing purposes, my wife and I don't go to such extremes. We top up our packs at home before heading out, and this gives us plenty of runtime. Sometimes we even do it the night before if we anticipate an early start, and we still have a good bit of charge available.

However packs that have been left off the charger for several days show significant evidence of self-discharge. They aren't dead by any means, but there is a noticeable drop in runtime and initial punch.

  • Like 1

Share this post


Link to post
Share on other sites

haven't seen any "Low Self Discharge" NiMH cells made in subC size yet :)

they're all the rage in AAs & AAAs... Sanyo used to make Enerloop Cs & Ds but think they've been discontinued since Panasonic bought Enerloop :(

Share this post


Link to post
Share on other sites

What is the recommended "storage" voltage or charge for old Nimh and NiCad packs?

I've recently started storing my Lipos at 3.8v / cell based on what I've read here.

Share this post


Link to post
Share on other sites

NiCads like to be stored discharged. I seem to remember 0.3 volts per cell as being optimal, but it has been a while so you might want to confirm this.

NiMH packs are the opposite - they like to be stored charged. We simply charge ours normally after a bashing session, and pack them away once cool. This seems to work well - even our older packs are still going strong.

Share this post


Link to post
Share on other sites

I had a couple of Orion nimhs and they used to drop 500mah in a week, when charged at 2c, whether they would continue to drop at that rate, I very much doubt.

I also wonder if charging them at a low rate, would slow the discharge rate.

Share this post


Link to post
Share on other sites

Thanks for all the replies.

I had some quite poor run times when I took a car out the other day. Two of my packs are second hand, two are newish, they were all charged about 10 or so days previous to my trip out. The above info suggests my run times we're perhaps correct.

I will try to do a couple of runs with freshly charged packs this weekend :)

Share this post


Link to post
Share on other sites

NiMHs straight off the charger are noticeably more punchy.

Share this post


Link to post
Share on other sites

Do Nimh packs have a 'memory', ie if a bashing session only uses half the charge, should it be discharged fully and then recharged and stored until the next run?

Share this post


Link to post
Share on other sites

After reading in the precious internet, i mixed up the terms nicd & nimh....

....a few weeks ago I charged all of my nicd batterypacks for storage because I thought in the past years I was handeling the batterys the wrong way. (The last years I stored them discharged...)

Is there anything I can do to rescue them?

Share this post


Link to post
Share on other sites

NiCads are usually quite resilient. Cycling them a few times often brings them back to life.

Share this post


Link to post
Share on other sites

For peak performance, NIMH need to be run hot off the charger, and stored discharged. The problem is they can't be discharged too far, and they cannot sit too long without maintenance. This is where dischargers like the Integy Reactor-20 and equalizers like the Trinity D-90 come in handy.

A nimh cell likes to be discharged fast down to 1 volt per cell under a 20 amp or higher load (this can be done as a whole pack) . Then a slow discharge of each individual cell on an equalizer tray down to 0.9v under a light .5 to 1 amp load. As soon as the equalizer cuts off, the cell's voltage will spring back to ~1.22 volts and sit there. This state will buy you the most time for storage (about a week) while maintaining the packs peak performance. When you pull the pack out and check the voltage each cell, they should still be above 1.0v to 1.2v). If you are down at 1.0v, you will either charge & run the pack hot, or cycle the pack and put it back into storage.

NIMH should NEVER go below 1.0v resting voltage or they will be irreversibly ruined. You may still be able to force them to hold a charge, but the pack's internal resistance will shoot way up and performance will drop like a rock.

PS: Nimh suck! All that silly effort will only result in about 3 months of peak performance.

Share this post


Link to post
Share on other sites

 

NiMH packs are the opposite - they like to be stored charged. We simply charge ours normally after a bashing session, and pack them away once cool. This seems to work well - even our older packs are still going strong.

 

So, I've found out I have NiCd & NiMH batterys...

I've understood that you store NiMH batterys charged, but before you run them after a long storagetime, do you discharge them & charge them or do you charge them without discharging them?

Share this post


Link to post
Share on other sites

I can't claim that our way of doing it is the best way, but we top them up before a bashing session, run them until the LVC kicks in, then charge them again when we get home. With this treatment, they seem to last well without losing capacity.

Share this post


Link to post
Share on other sites

For some time (think eneloop) there are low self discharge nimh aa/aaa cells - nowadays more manufacturers offer them, just not under any specific name.

How come I didn't see rc nimh packs with low self discharge cells?

Share this post


Link to post
Share on other sites

I think the reason may be that low self discharge cells have slightly less capacity for their size than regular NiMH cells. They might be also less suitable for high current operation, although I don't know if this is true.

Share this post


Link to post
Share on other sites

NiMH packs like to be stored charged.

For peak performance, NIMH need to be stored discharged.

Hi guys which is the right way to store nimh batteries then? I currently run them (until they come to a stop) and then discharge them as much as I can by running the car off the ground at full throttle until the wheels stop. I haven't got a discharger.

Then store them discharged until I need them again, recharge and then run them straight away to get the most out of them.

They seem to be doing well.

Share this post


Link to post
Share on other sites

I used to run NiCads exclusively. My first NiMH was a flight pack for one of my model planes. Not knowing any different, I treated it like my NiCads, and stored it discharged. When I came to charge it a couple of weeks later, it was ruined. It wouldn't hold a charge at all.

Since then I have been storing NiMH charged. They may not be at their absolute peak of performance as a result, but they don't seem to be going downhill either.

Share this post


Link to post
Share on other sites

Hi guys which is the right way to store nimh batteries then? I currently run them (until they come to a stop) and then discharge them as much as I can by running the car off the ground at full throttle until the wheels stop. I haven't got a discharger.

Then store them discharged until I need them again, recharge and then run them straight away to get the most out of them.

They seem to be doing well.

If you re-read what I wrote you will see. If you want "peak performance" store them discharged. But to do this, you need to monitor each individual cells voltage weekly.

If you don't want to mess with that, get yourself a Tekin battery nurse to store your packs. It will maintain the voltage for you.

If you don't want to do that. Then charge your pack on a low amp rate (probably 500mah) till it's about half full. Then store it like that. That will buy you extra time. The problem is your cells will develop memory from doing this. It will effect the performance of the pack later on.

Share this post


Link to post
Share on other sites

I've found that NiMh batteries are absolutely infuriating to maintain. I've tried multiple chargers and it is always 'hit or miss' how they will perform on any given day. I can usually get about 6-10 months out of one before they just fail to hold a charge. Last night I trashed a ruined Venom nimh because my peak charger apparently overcharged it too many times. I don't seem to have the overcharge/overheat issue with Nicads.

Nicads seem to hold up better and they are dirt cheap (I recently bought a duratrax onyx 1800mah for under $10), but of course they don't have the punch or capacity of a nimh in pristine condition.

I'm asking Santa for Lipo's, of course I'll need lipo alarms for my 3 models since Tamiya doesn't feel the need to make their ESC's Lipo compatible =)

Share this post


Link to post
Share on other sites

A charger with temperature sensor (the now so common accucel 6/imax b6 usually have the temperature port, and sensors are avaiable) should more or less prevent overcharge (since on nicd/nihm overcharge translates to very hot pack)

Also a good ideea to use while charging lipo packs

"This temperature sensor supports Accucel series/MEGA series/Quattro/IMAX B6 chargers"

http://www.hobbyking.com/hobbyking/store/__76277__Turnigy_Temperature_Sensor_for_Battery_Charger_EU_Warehouse_.html

76275.jpg

Share this post


Link to post
Share on other sites

Lipo should never get warm on the charger. The way the charger operates prevents heat from building inside the pack. In fact for max performance, some people actually use pack warmers to reduce the internal resistance and gain extra punch. Especially during colder outdoor racing events.

MMWarmingTray.jpg

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×
×
  • Create New...