Batteries / battery care for Tamiya MFU

[Mad Ax 9329]

I thought my days of NiCd and NiMH battery charging were long gone, but getting into rigs has given me cause to get back into the fun and games of long charge times, short run times and bizarre battery dumping characteristics.

I had hoped that running the packs in low-draw applications like Tamiya rigs would mean they last longer, but this weekend I had my first casualty: a 1800mAh NiMH that came new with a Maverick RTR a few years back.  For a long time the battery went unused - I was using LiPo for my fleet and it almost got taken to the recycling centre, until I started building my first rig.

In all its life it's had a handful of charges.  Definitely no more than 50 and probably no more than 20.  When I first started trucking I'd get best part of a day out of two packs - I didn't even bother to bring a charger.  I'd bring a third pack but not use it.  Only this weekend I had two rigs, not one, so although only one was being driven at a time, they were both on the layout powered on and waiting for an engine start command.  The Maverick pack in my new Globe Liner dumped after only a couple of hours and very little actual driving.  I've put it on the charger but it's hitting peak detect at way under full charge and isn't going up when left on a trickle, so I can only assume it's lost a cell.  I tried a full cycle on it but that didn't help.

So: what's the best way to care for packs when used in rigs?  Generally speaking I'll come home with a handful of half-discharged packs.  Up to now, these have hung around on the shelf until the next meet comes around, and they get charged up the day before the event.  I don't put them on an aggressive fast charge but if it would prolong life I could always start charging them earlier so I don't have to charge at 1C.

FWIW, I tend to keep one rig on the shelf with a pack in it at all times: my shelf is a little like a mini-diorama and it's nice just to put the lights on when I'm working in the dark some evenings.  Generally whatever pack I have in the truck when I come home stays in it until the next outing.

Should I be discharging my packs after every event?  If so, how low should I take them, and at what current should I discharge them?

Should I recharge them before storage or store them empty?  What's the best way to charge a NiMH for prolonged life (as opposed to boosting them for max punch or short charge times)?

Should I trickle-charge a NiMH or is that bad?  I always thought it was OK to trickle them like a NiCd but recent reading suggests maybe that's not so good an idea...

What's a good brand that will give me reasonable runtimes and good overall lifespan without breaking the bank?

Thanks

[Backlash 481]

This seems to be a common complaint when using the Tamiya MFU's with NiMh batteries.. 

IMHO, the reason that many people are having issues with NiMh batteries these days is that a lot of the batteries that are coming out of China are of low quality, and they are not really designed for the high charge and discharge rates that they will see when used in an RC application... All of the big name brands that we used to see with RC batteries like Sanyo, Panasonic, GP, Intellect (IB) etc are all gone...

The big issue with NiMh cells is that they self discharge when they are not being used, so this is why it is suggested that you should leave NiMh batteries with a partial or storage charge when not in use... The other problem is that the cells don't all self discharge at the same rate, so if you leave a Nimh battery on 50% storage charge, after a month you may find that some cells have lost 10% of their charge, where others are completely flat (discharged)..  This then causes the pack to become out of balance, and this is where you can damage the battery... If you try and discharge the unbalanced battery, the low cells can be over discharged, and their polarity will be reversed, which causes damage to the cells.. If you try and charge an unbalanced battery, the cells that still have a higher capacity will be over charged which again will cause damage to those cells... The only way around the problem is to individually discharge each cell after storage to balance the battery before recharging it.. Companies like Novak made a discharge balance trays (smart tray) that were designed to individually discharge the cells in NiMh ladder packs down to a preset voltage before you recharged the battery pack, but there are not many options if you are using stick packs...

I use 7.4Volt, 2S LiPo's in all of my trucks, even those fitted with an MFU, and I have not had any issues thus far... From what I have read on the various Facebook RC Trucks groups, there are quite a few others that use LiPo batteries as well......... BUT, I have also read or been told by a few members that they have "Blown up" their MFU by running a LiPo battery, although they could not tell me what or how the MFU failed....

The MFU's do NOT have a low voltage cutoff for LiPo batteries, so I just run a LV alarm on the balance lead.....

[Backlash 481]

As for brands... As you may have guessed, I have not bought a NiMh battery pack for quite a while, so giving you a personal recommendation is difficult, but....

I recall that some members on here have reported good results with the Vapextech batteries.... http://stores.ebay.com.au/Vapextech-Batteries-and-Chargers_72v-Battery-Packs_W0QQLHQ5fSellerWithStoreZ1QQLHQ5fTitleDescZ1QQ_fsubZ4QQ_sasiZ1QQ_sidZ121222805QQ_trksidZp4634Q2ec0Q2em322

I have also been told that the Turnigy NiMh batteries sold by HobbyKing are not too bad.. https://hobbyking.com/en_us/batteries/nimh-nizn-batteries.html

[berman 5315]

My yellow MAN with the mfc-03 ran a Team Orion nimh pack up until the time before last that I used it. It has either dropped a cell or over charged one as backlash pointed out above, it doesnt take a full charge and after it has been charged I install it in the truck and the truck shuts down under any load so I have finally changed it to lipo (the last of my trucks) and wont go back.

 

[Mad Ax 9329]

Thanks for the advice - I was thinking I'd prefer NiMH for the trucks but maybe now is the time to go LiPo.  I'll probably buy some new LiPos purely for use on the rigs, as they'll get very different usage to those in my other stuff.  Maybe I'll keep my classic Tamiya NiCd 1300mAh that came with my Full Option TGX for shelf duty, as it seems to be a really solid and reliable battery and is quite happy being plugged in on part-charge for ages.

I do have some LiPo alarms but I'll have to test them carefully in a controlled environment, preferably a padded room.  I had issues when running a pair of alarms on a pair of 2S stick packs in a Traxxas EVX2-powered Clod Buster - every time the alarms fired the Clod would go out of control.  I don't want my rig shooting off across the layout and doing massive damage to someone else's airbrushed pride and joy  

[Backlash 481]

6 hours ago, Mad Ax said:

Thanks for the advice - I was thinking I'd prefer NiMH for the trucks but maybe now is the time to go LiPo.  I'll probably buy some new LiPos purely for use on the rigs, as they'll get very different usage to those in my other stuff.  Maybe I'll keep my classic Tamiya NiCd 1300mAh that came with my Full Option TGX for shelf duty, as it seems to be a really solid and reliable battery and is quite happy being plugged in on part-charge for ages.

I do have some LiPo alarms but I'll have to test them carefully in a controlled environment, preferably a padded room.  I had issues when running a pair of alarms on a pair of 2S stick packs in a Traxxas EVX2-powered Clod Buster - every time the alarms fired the Clod would go out of control.  I don't want my rig shooting off across the layout and doing massive damage to someone else's airbrushed pride and joy  

Its not a bad idea to have an old NiCd battery for testing..  NiCd's don't seem to be as bothered if you run them completely flat (although they can still suffer from cell reversal) so its probably better to have the older 1300 mAh NiCd pack in the truck for shelf duties anyway... ( I would NOT leave a LiPo battery sitting in the truck..)

The low voltage LiPo alarms that I have are just that, an audible alarm with LED indication lights that sound and light up when the LiPo's voltage gets to about 3.3 Volts per cell.. I'm not sure how one would or could cause your Clod to go crazy, unless the ones that you have put out some type of weird RF signal when they activate??

LiPo batteries do hold a higher voltage throughout the discharge cycle, and then the voltage drops suddenly at the end of the cycle.. I'm wondering if the reason why your Clod goes out of control is because the alarms are causing some type of interference, or if its more because the battery voltage has suddenly dropped below what the ESC and receiver need to operate, and that is causing the receiver to lose contact with the transmitter..

You can see in the diagram below how the curves for the LiPo batteries are quite linear (flat) until they get to around 6.8V (3.4V/cell), and then the voltage drops suddenly.. You can also see the voltage of the NiMh battery (blue) Drops away quite early on in the discharge cycle..

 

[Mad Ax 9329]

36 minutes ago, Backlash said:

The low voltage LiPo alarms that I have are just that, an audible alarm with LED indication lights that sound and light up when the LiPo's voltage gets to about 3.3 Volts per cell.. I'm not sure how one would or could cause your Clod to go crazy, unless the ones that you have put out some type of weird RF signal when they activate??

LiPo batteries do hold a higher voltage throughout the discharge cycle, and then the voltage drops suddenly at the end of the cycle.. I'm wondering if the reason why your Clod goes out of control is because the alarms are causing some type of interference, or if its more because the battery voltage has suddenly dropped below what the ESC and receiver need to operate, and that is causing the receiver to lose contact with the transmitter..
 

well, that's exactly what I thought - in theory the alarms are just loud buzzers.  But it seemed to be that when the alarm activated the clod would go out of control, temporarily hitting full throttle so that it flipped over.  I run a short wheelbase on a custom chassis so it flips over very easy.  Once upside down, the motors and servos started glitching like crazy while both alarms were intermittently sounding and not sounding.  With 4S power and wheels wider than the body, it started dancing all around the street almost faster than I could catch it 

In fact the alarms were coming on way earlier than anticipated - less than a minute into a run.  I had only just finished the chassis and was testing it with some older 4.2Ah stick packs, not the 5Ah squares that I had planned to run it on longer-term.  It's possible the ESC was drawing more current than the batteries could provide, causing the voltage to drop off enough to trigger the alarm - I'm not really sure what happens to a LiPo if you draw more burst current than it can supply (and in all honesty I didn't think the Traxxas Titan 550s were that hungry).  So then it's possible that the voltage drop-off would cause the radio to glitch, causing more full-throttle, causing more drop-off, causing more glitch.  So the alarms might have been a symptom rather than a cause.

All I can say for sure is that I didn't want to risk my Clod or the cars parked along that road any more, so I went right inside and ordered a new EVX-2 with LiPo cutoff

 

[Backlash 481]

On ‎6‎/‎22‎/‎2018 at 2:05 AM, Mad Ax said:

well, that's exactly what I thought - in theory the alarms are just loud buzzers.  But it seemed to be that when the alarm activated the clod would go out of control, temporarily hitting full throttle so that it flipped over.  I run a short wheelbase on a custom chassis so it flips over very easy.  Once upside down, the motors and servos started glitching like crazy while both alarms were intermittently sounding and not sounding.  With 4S power and wheels wider than the body, it started dancing all around the street almost faster than I could catch it 

In fact the alarms were coming on way earlier than anticipated - less than a minute into a run.  I had only just finished the chassis and was testing it with some older 4.2Ah stick packs, not the 5Ah squares that I had planned to run it on longer-term.  It's possible the ESC was drawing more current than the batteries could provide, causing the voltage to drop off enough to trigger the alarm - I'm not really sure what happens to a LiPo if you draw more burst current than it can supply (and in all honesty I didn't think the Traxxas Titan 550s were that hungry).  So then it's possible that the voltage drop-off would cause the radio to glitch, causing more full-throttle, causing more drop-off, causing more glitch.  So the alarms might have been a symptom rather than a cause.

All I can say for sure is that I didn't want to risk my Clod or the cars parked along that road any more, so I went right inside and ordered a new EVX-2 with LiPo cutoff

 

If both the 4200mAh LiPo batteries were fully charged, then it seems strange that the alarms started coming on less than a minute into the run... I seriously doubt that the batteries were flat after such a short time, so you could always remove the alarms and give it a short run (in an open area) without them to see if they were if fact the problem...

Did you try and check what the preset voltage is/was for the alarms... Some are preset to a fixed value (eg. 3.3V/cell) but some other alarms (eg, some of the ones with a digital voltmeter) you can adjust the voltage that the alarm sounds, so perhaps the alarms cutoff voltage is just set too high??

Keep in mind that if you are running the motors in parallel, then it is effectively the same as running a single motor that is half the turns of the motors that are connected in parallel.. (eg, with two 12 turn motors connected in parallel, the current drawn would be similar to running a single 6 turn motor as far as the battery and ESC are concerned..)