v28 pack add-on to Li-Po 24V?

[redline8400]

Hello everyone,

This is my first post since the old Vis forum. I've recently finished my Cyclone 360w install on a Dahon Speed TR. It's all wonderful except for the fact that I'm missing a few Ah for my commute.

I've been looking to supplement my 9Ah Li-Po with a Milwaukee v28 pack. I know the RC guys state that the v28 recharges just like a Li-Po and was wondering if I could charge the v28 through my current Li-po charger.

Could anyone also show me a rough diagram of the wiring, I'm still a newbie when it comes to electrical configurations... There's naturally a BMS on the Li-Po pack and I was wondering if this would cause a problem if I added the v28 pack in parallel. I would still like to use the v28's BMS for discharging (as I think using my Li-po charger would not permit that).

Maybe it's just better to get a v28 charger as well since it will keep the individual cells balanced within the pack.

And if you're wondering why I haven't got a DrainBrain - it's coming in the mail!

I'm so glad to have found my old Vis forum!

Thanks for any help or comments.

Redline

 

[xyster]

I've been looking to supplement my 9Ah Li-Po with a Milwaukee v28 pack. I know the RC guys state that the v28 recharges just like a Li-Po and was wondering if I could charge the v28 through my current Li-po charger.

If your Li-Po charger charges, or has a setting for charging 7 cells in series, then yes you can charge the emoli's with the lipo charger. Li-po and Li-mn (lithium manganese, emoli) both peak at 4.2 volts, though the emoli can be drained lower, which is not something that affects charging.

Is your lipo pack really 7 cells? That's a rather unusual configuration for lipo which usually has 10 cells = 36 volts nominal and 6 cells = 24 volts nominal. Wiring a 7 cell emoli pack in parallel with a 6 or 10 cell lipo pack would not be good because the voltages will be way off! The BMS's may not like it either, and could fail to protect one or the other from overcharge by the other.

Assuming your lipo is not 7 cells, you could wire up your own emoli pack with the same number of cells in series as your lipo pack by buying the cells individually, or by cracking open a couple V28 cases, or by buying a custom pack from Mike here:
http://bigerc.com/index.php?cPath=21_26
This way you could use your lipo charger to charge the emoli's (which if left connected in parallel during charging will charge from the lipo charger anyway). Balancing may become an issue in this case. I know most people prefer a BMS for balancing, but if it was me trying this, I'd simply buy a cheap single charger and bring up the voltage of the low cells every 10 rides or so.

From everything I've read, 3ah of lithium-manganese does not require a BMS so long as you don't do anything super-stupid to it (which may fry the BMS anyway).

 

[TylerDurden]

I'll advocate for getting a v28 charger. "Keep it simple..."

Then, you can get some tools and your v28 pack works for you at home after commuting. (1hr charge)

Members jondoh and maytag both use v28 emolis for power: in-pack and custom respectively.

Ridgid tools makes 24v (6cell) emoli packs. Home depot sells a batt&charger for $199US (lifetime service)

 

[redline8400]

If your Li-Po charger charges, or has a setting for charging 7 cells in series, then yes you can charge the emoli's with the lipo charger. Li-po and Li-mn (lithium manganese, emoli) both peak at 4.2 volts, though the emoli can be drained lower, which is not something that affects charging.

My Li-Po pack is indeed 7s. If the BMS on the Li-Po cuts out the pack, does that put the v28 pack in an awkward position, aside from major voltage sag?

If I can indeed use my Li-Po charger to charge the v28, what sort of wiring schematic would you suggest? Would I charge both packs at the same time?

Thanks !
...

I'm the proud new owner of a DrainBrain with Speedometer and backlit LCD - yeehah!

 

[xyster]

My Li-Po pack is indeed 7s.

Interesting! And very good news as that will simplify things...

If the BMS on the Li-Po cuts out the pack, does that put the v28 pack in an awkward position, aside from major voltage sag?

If the BMS on the Li-Po cuts out, won't that most likely be due to low voltage due to the battery being run-down? If so, the emoli's will probably be run down as well...say your li-po packs cuts out at 3.7 volts per cell and so has about 20% charge remaining, then the v28 pack wired in parallel (I assume without diodes) will also be at 3.7 volts. Because it's Li-Mn and not Li-Co, it'll have a little more charge as a percentage of it's initial 3ah -- but not much charge compared to your bigger pack. In other words, if the LVC on the li-po trips, it's time to cut the trip anyway.
If the li-po BMS cuts out for some other reason, the emoli's will be fine because the cells can safely provide 35 amps, and are regularly pushed to 45 amps by the RC airplane people, and the v28 BMS (if I remember right) limits output to 30 amps.

If I can indeed use my Li-Po charger to charge the v28, what sort of wiring schematic would you suggest?

Assuming your lipo pack has a separate, dedicated charging port (meaning you don't need to unplug from the bike to charge), a straight across parallel wiring of the main power leads on the lipo, to the main power leads on the v28 pack, should allow the v28 pack to charge simultaneously and be balanced. I'm not absolutely positive this arrangement won't cause problems with one or the other's BMS.

If you want to be safe, disconnect the v28 pack for charging, and charge it with either a v28 charger, or with your lipo charger. I'm pretty sure (but not positive -- maybe jondoh or maytag knows) that the v28 charger simply provides a standard CC/CV ~29.4 volt charge through the V28 pack's main power leads, to the internal BMS which distributes the charge to each of the seven cells. If this is the case, I don't see why your lipo charger wouldn't charge the v28 pack properly since all it does is provide the same ~29.4 volts at some rate that is probably well within the range of what the emoli cells can tolerate and what the v28 BMS will accept.

Can you get a voltage reading on both your lipo charger, and v28 charger, and also tell us how long it takes your lipo charger to charge your lipos, and how long it takes the v28 charger to charge the v28 pack?

Sorry that most of this is not a straight-up "do X and it'll work for sure" thing. The only for-sure way is to charge each pack separately with a charger designed for that purpose. But depending on your needs and budget, that's not necessarily the best, most elegant or fun way...

 

[redline8400]

If the BMS on the Li-Po cuts out, won't that most likely be due to low voltage due to the battery being run-down?

Yes, that's what I was thinking, but I had a momentary bout of laziness...

Because it's Li-Mn and not Li-Co, it'll have a little more charge as a percentage of it's initial 3ah

I'm not sure I get this... Are you referring to the higher C rating of Li-Mn as compared to Li-Co?

Assuming your lipo pack has a separate, dedicated charging port

It does - XLR.

If you want to be safe, disconnect the v28 pack for charging, and charge it with either a v28 charger, or with your lipo charger.

I've made a (rough) drawing of what I think I understand is best and safest. It'll probably show up below.

I'm pretty sure (but not positive -- maybe jondoh or maytag knows) that the v28 charger simply provides a standard CC/CV ~29.4 volt charge through the V28 pack's main power leads, to the internal BMS which distributes the charge to each of the seven cells.

Should I PM them? Is that the best way to ask them a question?

Can you get a voltage reading on both your lipo charger, and v28 charger, and also tell us how long it takes your lipo charger to charge your lipos, and how long it takes the v28 charger to charge the v28 pack?

I don't have the v28's yet, but my Li-Po charges to about 29.3V. I installed my DrainBrain today and the pack cuts out at 22V. The pack normally takes 3-4 hours to charge and the charger is advertised as 5A, 24v output. I'm not familiar with how Li-Po chargers work, but if it states an output of 24v, then it must be a CC type, no?

And a last question. For shelf life, I know that leaving a pack at around 40% its capacity is best. What about daily use? Is it best to recharge right before using the pack, leaving it discharged most of the time, or the opposite - charging it completely as soon as I get home?

Thanks again!

Redline

[/quote]

 

[xyster]

I'm not sure I get this... Are you referring to the higher C rating of Li-Mn as compared to Li-Co?

No. I'm referring to the slightly higher accepted nominal voltage of Li-Mn (3.8v) cells compared to Li-Co (3.6v or 3.7v). If you look at a discharge curve, the li-mn has a little more energy left below the 3.7v point than li-co (at least that's my reading of charts I looked at some time ago). Li-mn I believe can be safely discharged to 2.5 volts; with a good BMS, li-co is ok to 3.0 volts (though it'll shorten their life to run them below 3.75 volts -- the ~80% li-co DoD point).

Should I PM them? Is that the best way to ask them a question?

That's what I'd do.

I don't have the v28's yet, but my Li-Po charges to about 29.3V. I installed my DrainBrain today and the pack cuts out at 22V.

Ok, so your main pack's definitely 7 cells in series, operating from 4.18 volts peak, to 3.14 volts trough.

The pack normally takes 3-4 hours to charge and the charger is advertised as 5A, 24v output. I'm not familiar with how Li-Po chargers work, but if it states an output of 24v, then it must be a CC type, no?

Most are like SLA chargers -- CC to a point, then switches to CV as the current trickles down to a cut-off amount of like 500ma depending.

And a last question. For shelf life, I know that leaving a pack at around 40% its capacity is best. What about daily use? Is it best to recharge right before using the pack, leaving it discharged most of the time, or the opposite - charging it completely as soon as I get home?

I don't know that anybody's studied maximizing li-co calendar and cycle life in great depth, but this is what I've gathered from the Tesla motors blog, batteryuniversity.com, and other sources, and it's how I try to treat my li-co pack:

- minimize time spent above 4.15 volts. If possible, charge only to 4.1 (90% charge) or 4.15 volts (95% charge).
- minimize heat exposure, especially when batteries are also fully charged.
- Don't discharge below 3.7 volts.
- Maximize time spent at low temperatures (but not below 32F) and low, ~40% (~3.8v) charge state.

Your wiring diagram looks like it'll work great, and that by doing it this way, you won't need a dedicated charger for the v28 pack since it'll charge from the main pack.

The one question I still have is if the V28 packs normally charge through the same, main power leads that you're planning on connecting in parallel to your main pack. If yes, then the V28's internal BMS should do all the stuff it normally does including balance the cells, and you wouldn't necessarily need to crack the case at all (good for warranty ). If no, I'm not sure if the V28 BMS will balance the cells or not, which would mean some kind of work-around would be necessary -- no big prob, lots of ways to handle it.

If it was me, I'd give Jondoh and Maytag a PM, read through everything V28 related here and elsewhere, and start a new thread with just these kind of direct questions.