12S or 14S? what do you recommend?

[mxs]

Hi folks,

the first time I am venturing into Lipo RC packs and utilizing them to propel my bike. Stock the bike comes with 13S1P, 25Ah pack. I don't know what cell pouch they used. This pack is good for long distance, long hours riding. it works great. But it also weighs 10kgs and 90% of time I am carrying bike weight I never use. So I want to make a smaller battery, I can easily slip in instead of the stock one.

The battery needs to provide 120V cont. and 300A peak. System uses Kelly controller which can go up to 60V. The controller is running a custom setup where low voltage cut-off is at 37V, it will actually start dialing down power at 40.7V before it just cuts-off completely.

I am thinking of using Turnigy RC lipo packs, but am not sure whether to use 2 x 6S packs or rather 2 x 7S packs, 5Ah each. I know 7S will be under the 60V threshold and generally would be considered better, but how about the low end. 37V/14 = 2.65V ... too low for safe and long life of the cells? Also, to clarify, I would use BMS, but just for charging purposes, not for discharge with these sort of burst amperage involved. I might hookup the BMS to a contactor, so the contactor would disconnect the main power out from the pack the moment the BMS LVS cut-off will basically disconnect the contactor energizing voltage. But this is quite new to me, so it would be down the road .... I can monitor the pack by voltage monitor, once the idle voltage goes under something like 3.1V/per cell, I would just recharge the battery. It's no big deal to me. This should save me about 8kgs of weight, which is really dramatic and welcomed for many reasons ...

So, would you recommend 6S or 7S packs? I know 6S will ultimately provide less power (both packs have comparable maximum discharge rates), but I don't think I will miss the little difference in that particular low speed application. And 6S seems to provide better cell cut-off voltage of 37/12 = 3.08V. What do you think?

Thanks for your help.

 

[john61ct]

A. nominal range
B. do not discharge anywhere near
C. charge range

A / B / C

13S: 47-48 / 39 / 53-54
12S: 43-44 / 36 / 49-50
14S: 50-52 / 42 / 57-58

The battery needs to provide 120V cont. and 300A peak.

You mean 120A?

> The controller is running a custom setup where low voltage cut-off is at 37V, it will actually start dialing down power at 40.7V before it just cuts-off completely.

You want to keep that, not reprogram it?

And you don't ensure you never hit that yourself, or you want to let it do its thing?

If the latter, 14S longevity will tank, ideally you never touch 3.4Vpc, 3.2V briefly at very high C-rates.

Will work well for 12S, but the former strategy is better and eliminates that factor.

 

[mxs]

Yeah, I meant 120A ...

The bike is still under warranty, so reprogramming the Kelly box is not something I'd do before warranty expires. I'd prefer initially to rely on the controller to cut-off the battery at 37V. But I will recharge a lot earlier if need be ... I thought 3.1V was sufficient, but you are saying I should use 3.4 ideally?. I also do realize that to go off the idle voltage is not ideal, but it will have to do in the beginning. I will try to utilize BMS via the main battery contactor to cut the power off which would be a lot more elegant solution in my opinion.

So, I guess without re-programming the Kelly controller, you'd suggest to go with 12S, unless I could go 14S and utilize the BMS to read the voltage? Mind you I have yet to see a BMS for 14S which allows me to set the discharge LVS to something other than they normally come with (I have yet to see anything over 3V ....) ... can you recommend any BMS for that application and functionality?

Thanks very much

 

[john61ct]

My approach with cheap BMS (not costing hundreds of dollars) is they are final-layer failsafe protection to prevent pack destruction.

Never to be activated in normal usage cycling except (maybe) balancing functionality.

An amps and voltage readout plus Ah counter should guide the user, familiar with the usual cycling patterns

if actual LVC is desired, put in an adjustable one, maybe alarms set a bit higher

goal is to stop discharging long before the BMS LVC point is reached.

Quality charger, adjusted to stop long before the stressful maximum SoC is reached.

Otherwise killing battery longevity.

 

[motomech]

No offense intended, but you might want to do some more research, as you have left a lot of blank spots in your query. LiPoly is a BIG subject.
Generally, one picks the Voltage to get the desired top speed for the motor/whl. size combo it is to be mated with. On my b low-powered bikes, the difference is 3 to 4 mph.
Nom. Voltage for 12S is around 46 Volts.
Nom. voltage for 14S is around 54 Volts.
LiPoly cells should not be dis-charged below 3.65 V/cel, at with point they can start to stray.
LVC for 12S - 41 to 42 Volts
LVC for 14S - 50 to 51 Volts.

The first thing I will say is, your proposed pack size of 5 Ah is way to small, even for an aux. pack.
With the discharge rates you are indicating, I would roughly guesstimate you would see around 1 mile/Ah, or 5 miles if you discharge most of the pack, something that is a bad idea.
Since LiPoly is about 1/3 smaller and lighter than round cell chemistries, you should run at least a 10 Ah pack since it will still be small and in that way, you won't have to work the bricks that hard.

The next thing you need to consider is what is available. Have you scanned Hobby King lately? Not much, as a lot of stuff has been discontinued. Very little in the way of 7S bricks, especially in the large bricks (8Ah to 12Ah), which should be what you are looking for.
There is so much about living with LiPoly I could go over, but suffice to say, it's always better to use as few bricks as possible, to facilitate charging and keep the wires to a min. 2S/2P is good and 2S/1P is even better. If you are going to bulk charge, 3P and up gets complicated and if one is breaking the string for an RC charger, 3S and up gets complicated.
Speaking of charging, you haven't even mentioned it, charging is everything w/ LiPoly. Are you going to "bulk charge" and passively ck and balance the cells or are you going to "break the string" and use one or more RC balance charger(s)? Only a few of them do 7S and they tend to be be pricey.
LiPoly, while I consider it safe enough, can be rather time consuming, in fact, it can be a hobby in and of itself.
Given that you will have to purchase a charging set-up, you might be better off to get another Li-Ion battery, just smaller.
If you want to press on w/ LiPoly, let me know and we can go over more of what you need to consider.

 

[mxs]

I agree as far as the research, hence me trying to learn some things here. So your honesty and input is certainly appreciated.

The bike is a trial bike ... so you don't shoot for long miles range or top speed ... you basically hop from rock to rock, do a lot of wheelies stoppies etc. You play for 30 mins and your arms and legs are done ... so you die well before 5Ah battery would, is my guess ... LOL

I don't want to go to 18650 packs, which I have experience of making, because you end up with similar weight as stock once you build up the Ah and voltage ... no point doing that if I cannot get the weight of the pack significantly down compared to the stock. I can as well stick to the stock battery. The only solution is Lipo, otherwise there's no project as far as I am concerned ...

Turnigy 6S or &S 5Ah pack offers 60C discharge cont. ... which I don't believe, so I take 30C and 60C burst .. which should give me safely discharge I need. Am I wrong? I could go after higher Ah, if you think it is safer or necessary ... Just costs more money right?

Regarding charging .... I plan to use 2 equal packs to get the voltage I need, I don't want to get into 3 of anything .... I have PowerLab 6 .. which will require to take the packs out. No biggie, but would prefer setup which can keep the battery in the bike. Simpler ... but I hear you, perhaps more difficult to make it as safe.

I appreciate your input.

 

[mxs]

BTW, the bike came stock with Delta Q charger ... loaded with specific profile for the 13S 25Ah battery in it.

 

[john61ct]

Turnigy 6S or &S 5Ah pack offers 60C discharge cont

Even 30C is still unrealistic except for sub-second bursts. Just because a battery **can** manage high C-rate bursts, doing that really kills longevity.

For any longer "continuous" design / choose for 8-10C max, and even that for say half-minute bursts.

And forget the old charger. Your PL6 will be fine.

 

[999zip999]

? What charger doyou have ??

 

[mxs]

Turnigy 6S or &S 5Ah pack offers 60C discharge cont

Even 30C is still unrealistic except for sub-second bursts. Just because a battery **can** manage high C-rate bursts, doing that really kills longevity.

For any longer "continuous" design / choose for 8-10C max, and even that for say half-minute bursts.

And forget the old charger. Your PL6 will be fine.

Half minute burst is a lifetime in trial ... you really are blipping a throttle, moreless 0-50% sometimes 75%, but very seldom. You are doing a ton movement with your body, not just relying on the power of the drive-train. I'd say 1-2s of WOT or most of the time not even that ... more like 50-75% throttle application the most. Considering that, you would still recommend that I take the Turnigy posted continuous discharge and slash it by factor of 3? I don't mind to build some safe factor into the pack, I just don't want to overbuild it, considering the application is nothing close to what normal bikes would have seen.

I know the PL6 will be fine, but it means to take the packs out every time it needs charging. Not end of the world, just not as simple as plugging it in, and unplugging it when it's done.

 

[mxs]

? What charger doyou have ??

PL6

 

[john61ct]

My priority is lifespan.

You can go lighter, smaller and just replace more frequently if performance is yours.

And a bulk charger can be just a PSU with the right voltages.

In fact I have a pile of MeanWell HLG's I've been snagging off eBay, call it a poor man's Grintech Satiator.

Can crimp whatever connectors you want, either side. IP rated for outdoor use, very small and rugged, can even mount on a bike.

The 240-54A version can be turned down to 50.4 or 50V minimum, up to 57V, so perfect for 13S or 14S as well.

Current a bit over 4A per unit, can be stacked to charge faster, current turned down if you want to baby the pack.

$40 each including shipping.

 

[john61ct]

Also check out this discussion for high performance LiPo packs, cheap vs top-notch, very relevant to your use case

https://www.rcgroups.com/forums/showthread.php?p=42898563

 

[mxs]

Thanks for the suggestion regarding the PSU as a charger, I might take you up on that.

I am reading the thread about the Lipo packs as we speak.

 

[mxs]

Long thread ... .... I also got to the page with extensive test charts. My notes out of it ... all manufacturers lie, especially regarding C ratings ... not shocking. For 6S 5Ah packs, you either go for the best $ per Wh and buy Turnigy HD and hope for the best taking into account the actual C rating achieved in tests ... also the tester has been very suspicious that the results were too good to be true. Or you get higher QC brands for more money, something like HobbyStar pack or Dinogy, which will easily be twice the money of Turnigy HD as far as I can see. Tough call as always. Also, there’s a difference not getting advertised C rating without overheating, and not getting advertised C rating and blowing up in smoke right after. Decreasing longevity is one thing, but fire is all another ...

Sounds like Turnigy HD, Turnigy Nano-Tech +, HobbyStar .... depends how much money I want to spend. Maybe the Nano-Tech plus is a good middle of the road choice able to deliver 40C cont. which should suite me well.

What brand do you recommend for around 100USD budget on 6S 5Ah pack?

Thanks