Batteries - Low Self Discharge?

[Gebbly]

Next stop in my R/C hobby education today was batteries. One of the big decisions was NiMH or LiPo. After weighing up all the pros and cons I think for my first build I like the easier maintenance and safer characteristics of a standard NiMH. It will  be going in my GF-01 Wild Willy 2 project which will be all about the building experience and then trundling around the garden so I'm not trying to squeeze as much performance as possible out of the car. For the moment safer and easier = happier.

One of the annoying things about the batteries generally for me is their rate of discharging when just sitting on a shelf. I may build the car and then just go for the occasional pootle in the summer and then decorate my shelf in the winter. I ran across the term "Low Self Discharge" where some batteries are very good at holding on to as much of their charge as possible when not in use. I have found various AA LSD (Low Self Discharge) batteries.

Has anyone ever come across a standard NiMH battery pack that could fit in a Tamiya car (in my case a battery compartment 141mm x 47mm x 25mm) which had LSD?

[Willy iine]

I don’t think mine are, but with NiMH, I usually keep a few charged at all times and when needed just peak charge it just before I run it especially if it was sitting longer than a week.  

[alvinlwh]

While I cannot advise on the LSD feature of RC battery packs, what I will say is just store the packs fully charged to reduce your charge-to-run time when you need to top them up before use. 

While I had seen a number of lengthy posts of views (nonsense) on storage charge for NiMHs, broadly speaking, straight from the manufacturers, these cells store just as well full charged or drained. The only recommendation is to cycle them once a year (hopefully you will use them more frequently than that) and a few cycles will restore a pack that had been deeply discharged. 

https://www.pmbl.co.uk/blog/how-to-store-nimh-battery-packs

Also also to store them at reasonable temperature, which basically means room temperature is fine. 

[Mad Ax]

I use LiPo in most of my cars, but some of my big rigs still use NiMHs.  I tend to keep them fully charged and ready to go.  I don't use the rigs very often (actually only once since the pandemic started due to the clubs closing down), but there's always charge there if I just want to switch on a rig to test something or run it on the patio.

The opposite applies to LiPo - never store them fully charged, as it damages them.

[Gebbly]

Great advice thanks everyone. The storage of LiPos did seem a bit more fiddly. Its good to know I can just fully charge a NiMH and leave it on a shelf and as Alvinlwh advises, if its been a while just do a few charge cycles to refresh them.

[Willy iine]

@Gebbly the only thing I watch out for on NiMH is not to drain sport packs beyond 30%.  Since there is really no way to balance the cells individually on sport packs I try to keep away from discharging too far so that the cells remain relatively similar in voltage.   

Typically I keep about 50% remaining in the cells, let them rest overnight before recharging them again.  

[Gebbly]

20 minutes ago, Willy iine said:

keep about 50% remaining in the cells,

Handy tip thanks Willy iine. When driving how do you know how much capacity is remaining? Do you wire in some form of voltage sensor to the receiver to feed back to the controller?

I had never thought about letting batteries rest before recharging. I shall take this on board too.

[Gebbly]

Oh and an update on my searching for a NiMH LSD battery I did come across these

https://www.overlander.co.uk/nimh-batteries/panasonic-eneloop-aa-2000mah-7-2v-aurora-nimh-battery.html

Looks like overlander can make custom packs if needed but this pack is close as it is NiMH, fits in the battery compartment and uses eneloop cells which have great low self discharge. Shame its only 2000mAh and the JR connector could be a problem. I suppose I could get some JR connector to XT60 to connect to ESC? I'm game for trying soldering of connectors on things but I'm a little wary of going near a battery with a soldering iron.

But for my particular use case I could just keep 3 or 4 of these charged on a shelf somewhere and swap them out when running around the garden. I'll keep looking in case I find any anything with greater capacity though.

[Willy iine]

27 minutes ago, Gebbly said:

Handy tip thanks Willy iine. When driving how do you know how much capacity is remaining? Do you wire in some form of voltage sensor to the receiver to feed back to the controller?

I had never thought about letting batteries rest before recharging. I shall take this on board too.

I use a timer on my transmitter.  During the initial drive, I run 10 min and the let the battery pack rest over night.. then charge the battery the following day to see how many mAh the battery took in.  

That tells me how many cycles of 10 min runs I can do, and if the cycle is too long or too short, adjust the timer accordingly for each run until I can charge the battery by around half each time.  

[alvinlwh]

13 minutes ago, Gebbly said:

Oh and an update on my searching for a NiMH LSD battery I did come across these

https://www.overlander.co.uk/nimh-batteries/panasonic-eneloop-aa-2000mah-7-2v-aurora-nimh-battery.html

Looks like overlander can make custom packs if needed but this pack is close as it is NiMH, fits in the battery compartment and uses eneloop cells which have great low self discharge. Shame its only 2000mAh and the JR connector could be a problem. I suppose I could get some JR connector to XT60 to connect to ESC? I'm game for trying soldering of connectors on things but I'm a little wary of going near a battery with a soldering iron.

But for my particular use case I could just keep 3 or 4 of these charged on a shelf somewhere and swap them out when running around the garden. I'll keep looking in case I find any anything with greater capacity though.

Nonono, those are for a TX, which has far lower amp (C) requirements. AA cells has far lower power than the sub C cells of vehicle batteries and will do badly if used to power a 540 motor of a car. 

Sub C typically have a C rating of 3 - 15, typically about 10.

https://www.grepow.com/blog/battery-monday-maximum-discharge-rate-of-nimh-battery/

Whereas AA cells have a C rating of 0.25 - 0.5.

[Gebbly]

Ooh thats clever Willy iine. I'll definitely try that.

Oops, thanks for the warning Alvinlwh. Guess I still have lots of battery learning to do. Thanks for preventing me from making a big mistake there.

[SupraChrgd82]

Batteries with low internal resistance tend to dissipate a charge faster whilst in storage.  The downside to the low storage discharge rate is a low discharge rate (punch).  Not that NiMH has much punch compared to LiPo. But that’s irrelevant and I’m not going anywhere with that statement about punch.  The point is that higher mAh and performance-oriented NiMH have relatively low internal resistance and will self discharge comparatively faster than “general use” NiMH packs.

Batteries are chemistry.  What’s a catalyst in chemistry? Heat.  Batteries have the potential to discharge faster when warm.  The opposite is cold.  Store them in a cool dry place, maybe even the fridge, to slow that chemical reaction.

I’m with you though.  I have a stack of NiMH packs that are stored after being peak charged.

[Saito2]

8 hours ago, SupraChrgd82 said:

I have a stack of NiMH packs that are stored after being peak charged.

How long can/should they be stored in charged form before use?

[alvinlwh]

12 minutes ago, Saito2 said:

How long can/should they be stored in charged form before use?

Manufacturers recommend at a charge a year, but no matter what, they should be topped up before use due to the self discharge. 

https://www.grepow.com/blog/battery-monday-maximum-discharge-rate-of-nimh-battery/

[Mad Ax]

You may be overthinking the issue here.  There's absolutely nothing wrong with buying a generic hobby-grade NiMH pack and using it in your GF-01.  It will hold a charge in storage for a good while, and you can always top it off quickly before you go running if you've left it more than a months.  That will be way quicker than charging a depleted pack.  If you're not running a hot motor then the pack should last a while before it gets out of balance, which in my experience is the killer for NiMH batteries.

If you buy 2-3 packs of reasonable-quality batteries you should still have a fair amount of garden runtime without having to plug in the charger.  Run one until it's depleted, put it on charge while you run the second and third packs.  Depending on how hot you want to charge them, maybe the first pack will be ready by the time the third pack is dead.  Or maybe you'll find 3 packs is more than enough for garden bashing, so you can charge them all slowly and get more life out of them.

Sourcing high-spec cells and making customs packs is fine, racers used to do that back in the pre-LiPo days, but it's costly, needs specialist soldering / welding equipment, and I don't see any appreciable benefit for batteries that will spend most of their life on a shelf.

[Willy iine]

Yeah, for me I usually buy the cheap $15-20 NiMH packs sold on Amazon (3000-3500mAh range).   The higher priced ones in the same capacity are usually just a re-badge of the same(?) thing as they aren't using matched cells.   

They usually last a good 5 yrs or so.  

The oldest one in service in my NiMH batch is about 8-9yrs old and it's only a 2000mAh.    Still holds a charge well.

I don't race any more and drive only causally so they do just fine especially on silver or black can cars.  

[Gebbly]

I think I like your suggested way of doing things. After spending some more time searching I prefer the simple approach of just getting some basic batteries and full charge shelving them and then topping up the day before I want to use them. It was getting to be a very deep rabbit hole that I didnt want to be down as it was feeling more like work than fun.

Willy iine, since you drive GF-01s roughly how long do they run stock with a 3000mAh battery? 5 or 10mins?

[94eg!]

Ideally you should store your nimhs mostly charged. When removed from storage they should be discharged to flat. Then they should be charged just before use. Running them right off the charger will give a VERY noticeable boost in performance and runtime.  As mentioned earlier, the increased heat in the cells lowers their internal resistance which provides higher instant current. But Nimhs also loose a considerable amount of their stored charge within the first hour of coming off the charger. Cars will a lot more fun to drive this way.

[DTSCB]

Cries in the death of Nicad.  I have a few older cars (Losi) that only Nicads really fit in.  I still have a couple.  But they seem to be impossible to find today. 

[Gebbly]

Thanks 94eg! I'll remember that. I think I'm going to have to start an RC guide document on my PC with all the advice and tips I've been collecting.

DTSCB did Nicads have some characteristics that made them better for you than NiMH?

[Willy iine]

On 6/13/2022 at 8:02 AM, Gebbly said:

I think I like your suggested way of doing things. After spending some more time searching I prefer the simple approach of just getting some basic batteries and full charge shelving them and then topping up the day before I want to use them. It was getting to be a very deep rabbit hole that I didnt want to be down as it was feeling more like work than fun.

Willy iine, since you drive GF-01s roughly how long do they run stock with a 3000mAh battery? 5 or 10mins?

I usually drive two 12 min runs on the stock motor with a 3000mAh.    But I only drive mildly so I would run it just 10 min the first time and see how much charge it takes in to gauge where you need to set your timer at and how many cycles before charge.  

[DTSCB]

12 hours ago, Gebbly said:

Thanks 94eg! I'll remember that. I think I'm going to have to start an RC guide document on my PC with all the advice and tips I've been collecting.

DTSCB did Nicads have some characteristics that made them better for you than NiMH?

In the NiMH era, I've found the Nicad sub C cells are little smaller.  I have several older Losi cars that just don't fit any NiMH 7.2V packs.  To be honest, the car were picky about Nicad packs back in the day too.  Just worse now. 

[94eg!]

Yeah. Increased capacities of nimh saw sub-c battery sizes grow considerably. I found the small Orion 2400 lipo hard-pack fits perfectly in old ni-cad battery slots. So long as you have space for the bullet plugs.

[SupraChrgd82]

On 6/13/2022 at 6:02 AM, Gebbly said:

I think I like your suggested way of doing things. After spending some more time searching I prefer the simple approach of just getting some basic batteries and full charge shelving them and then topping up the day before I want to use them. It was getting to be a very deep rabbit hole that I didnt want to be down as it was feeling more like work than fun.

Willy iine, since you drive GF-01s roughly how long do they run stock with a 3000mAh battery? 5 or 10mins?

That’s exactly how I run my cars, mostly crawlers.  Batteries are charged and shelved, then topped off only if they sat for more than a month. Not sure if it the right or wrong way to go about it, but the system works. My family and crawl 1-2 times per month using a pile of 10 NiMH packs. We average 2 packs each and go about 2.5 miles.   I notice that the 5000mAh packs self-drain faster than the 3000mAh packs.

Regarding your 3000mAh, your runtime will depend on your motor (# of turns) and driving surface (grass vs hard surface). I can say that that I use 3000mAh Tenergy packs in several M-chassis cars on asphalt at 17t brushless and get 45-60 minutes of runtime.

[Gebbly]

3 hours ago, SupraChrgd82 said:

I notice that the 5000mAh packs self-drain faster than the 3000mAh packs

How do you find that out. Are there battery details that indicate this rate of self discharge?

45-60 minutes would be more than enough for me from a single battery. Its certainly worth taking note of, thanks. I would be running the standard torque tuned Tamiya motor (brushed) that came with my kit and mostly running it around my garden which is a mowed lawn.