Super fast charge

Batteries, Chargers, and Battery Management Systems.
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cheapcookie   100 W

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Super fast charge

Post by cheapcookie » Apr 11 2021 2:18am

Hey guys, I have a 12s battery identical to the one in my bike. Would it be smart to have some sort of current limit in the bms and just plug in my bike to the external battery when I get home? The external batt would be permanently connected to my charger. How would you guys go about implementing this? Is it worth it? It's a 18650 pack 12s6p currently charge at 75watts, but I guess I could go to 250-300watts easily.

Vbruun   100 W

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Re: Super fast charge

Post by Vbruun » Apr 11 2021 12:06pm

not worth, just buy a fast charger if that's what you want.

If you have a current limiting BMS, it will quickly open and disconnect the two packs because the current flowing can be much larger than what is acceptable for the cells.

If not, you will either set something on fire or brick your pack quickly.

john61ct   100 GW

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Re: Super fast charge

Post by john61ct » Apr 11 2021 12:28pm

Current **limiting** is not a function of any BMS I know of.

Most don't even have current protection, they just give a rating - usually too optimistic - and release the magic smoke if current is allowed to go too high.

Charging very fast can greatly reduce pack lifespan, especially in cool weather.

Better to stick to an hour charge or two hours even better, and have a second pack to swap out.

john61ct   100 GW

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Re: Super fast charge

Post by john61ct » Apr 11 2021 12:30pm

LTO chemistry is an exception, 10min charge NP

But much lower energy density, for a low speed dump truck or big bus maybe

markz   100 GW

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Re: Super fast charge

Post by markz » Apr 11 2021 2:12pm

Better to have way more battery then you need, to increase the safe C-rating for charging. Downside to that is the battery gets bigger, and heavier, and harder to place. If you need 1kwh battery (48V 20Ah) and you have a 8A charger then get a 48V 35Ah and get a 15A charger.

jonyjoe303   100 W

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Re: Super fast charge

Post by jonyjoe303 » Apr 11 2021 3:57pm

you can find 30a buck converters that can handle up to 70 volts input power. With a buck converter it will take the input power and reduce the output to the voltage you need, also the output amps can be whatever you set it for. Since the buck converter is cc/cv it behaves similar to a cc/cv charger.
The only problem with the buck converter is if the input and output voltages are very close to each other, you will not get high amp charging.
For high amp charging you would also need a boost converter to boost the aux battery voltage 10 volts higher, then reduce it with a buck converter to what you need and keep the max amps charging you need.
The 30a buck converters are in the 40 dollars range, the 30a boost converters are in the 30 dollar range. What you want to do (dc to dc battery charging) is possible but might ultimately require use of boost/buck converters. I use smaller 10a boost/buck converters to charge my 3s li-ion from my 4s lifepo4 or vice versa, and they work great for that.

But with any charging its always better to charge at lower amps, it keeps the battery from drifting out of balance, at high amps the bms might trigger to stop the charge too early.

picture of a 6a boost/buck converter I'm using for dc to dc charging. The ones you need for your situation are much larger and have giant heatsinks.
1 boost buck internals.jpeg
1 boost buck internals.jpeg (101.84 KiB) Viewed 379 times

markz   100 GW

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Re: Super fast charge

Post by markz » Apr 11 2021 4:33pm

You lose efficiency on those converters.

cheapcookie   100 W

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Re: Super fast charge

Post by cheapcookie » Apr 11 2021 9:29pm

Could I use a brushed pwm motor controller to modulate the current output ?

john61ct   100 GW

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Re: Super fast charge

Post by john61ct » Apr 11 2021 9:49pm

Get a charger (or PSU, or DC-DC converter) that allows you to custom-adjust both maximum setpoints, CV and CC current.

If your source is cheap grid power, don't need to be too concerned about efficiency.

Auto-termination is IMO much more important, safety first.

john61ct   100 GW

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Re: Super fast charge

Post by john61ct » Apr 11 2021 9:57pm

Since you have two identical batteries, just set them up to swap easily?

Better (safer simpler easier) to charge one pack at a time

but if you really want to parallel them, do so at the cell/group level 1S

so whatever you are using to keep the packs in balance can monitor at that level to prevent any group going too high.

Make sure that both packs are within a volt or so of each other before connecting

IOW each group within 0.1V.

And never mind watts, use amps for charging questions. Or C-rate, what is the Ah capacity of the packs?

cheapcookie   100 W

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Re: Super fast charge

Post by cheapcookie » Apr 12 2021 4:06am

The batteries can't be swapped, I guess my question is, can a battery be used as a charger. I know there are faster chargers. The question lies in using my second battery as a charger.

Vbruun   100 W

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Re: Super fast charge

Post by Vbruun » Apr 12 2021 4:43am

cheapcookie wrote:
Apr 12 2021 4:06am
The batteries can't be swapped, I guess my question is, can a battery be used as a charger. I know there are faster chargers. The question lies in using my second battery as a charger.
From a practical standpoint, it can't.

fatty   1 kW

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Re: Super fast charge

Post by fatty » Apr 12 2021 11:21pm

cheapcookie wrote:
Apr 12 2021 4:06am
The batteries can't be swapped, I guess my question is, can a battery be used as a charger. I know there are faster chargers. The question lies in using my second battery as a charger.
No, that's why one is called a battery, and the other is called a charger -- they're different.

Seriously though, you could parallel the batteries and they would rapidly equalize (though not "charge" because it would be uncontrolled), but there'd be a risk of exceeding the charge and discharge current at first, and as the voltage potential decreased, so too would current flow. So yeah, if you don't mind risking cell health, you could quickly dump maybe 1/4 or 1/3 into the empty pack.
You already have the packs -- try it outside once to see. If they don't get hot and you can top-balance them after, they're probably fine.

The correct solution though, as mentioned, is to make the packs swappable.
Don't take advice from:
there is no difference between a mean well CC/CV power supply and a device sold as a charger. they operate in EXACTLY the same way
Testing has demonstrated that ordinary rim brakes thermally outperformed all but the best disc brakes...You'll always add weight and cost, while not equalling the capabilities of comparable rim brakes, if you use discs

john61ct   100 GW

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Re: Super fast charge

Post by john61ct » Apr 14 2021 12:04am

You of course **can** charge from a 100% Full pack to an "empty" one

but then if they are the same Ah capacity, bith are now at 50%.

You need to (MUST for safety) limit the current transfered when initially paralleling them.

Both problems can be solved with a quality DC-DC **charger**, but I've not come across ones designed for that voltage range.

A well engineered DCDC **converter** with excellent current limiting circuitry (not just CP as in latching or hiccup style) might do the job.

None of this gear is cheap, also requires skillz to be safe.

So in this case I'm going to say , no you can't as a practical matter.

If you were going to sell such a product, hiring an engineer to cobble one together would be a pretty big investment.

cheapcookie   100 W

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Re: Super fast charge

Post by cheapcookie » Apr 14 2021 2:26pm

john61ct wrote:
Apr 14 2021 12:04am
You of course **can** charge from a 100% Full pack to an "empty" one

but then if they are the same Ah capacity, bith are now at 50%.

You need to (MUST for safety) limit the current transfered when initially paralleling them.

Both problems can be solved with a quality DC-DC **charger**, but I've not come across ones designed for that voltage range.

A well engineered DCDC **converter** with excellent current limiting circuitry (not just CP as in latching or hiccup style) might do the job.

None of this gear is cheap, also requires skillz to be safe.

So in this case I'm going to say , no you can't as a practical matter.

If you were going to sell such a product, hiring an engineer to cobble one together would be a pretty big investment.
Thanks John! I'm not going for practicality here, here to learn. Thanks alot, this clearly informs about the various options

fatty   1 kW

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Re: Super fast charge

Post by fatty » Apr 14 2021 5:08pm

john61ct wrote:
Apr 14 2021 12:04am
You of course **can** charge from a 100% Full pack to an "empty" one

but then if they are the same Ah capacity, bith are now at 50%.
Just for completeness, in the context of a "super fast charge", the empty pack wouldn't get to 50% in a reasonable amount of time, since current would decrease as a function of voltage differential -- which would decrease as the packs equalize.
john61ct wrote:
Apr 14 2021 12:04am
You need to (MUST for safety) limit the current transfered when initially paralleling them.
Again, in the specific context of a "super fast charge", I'd expect current limits are deliberately ignored with any charging method.
I'm an advocate of safety, but due to the dynamics of equalization (and because it would be supervised) I actually don't see this as being that dangerous.
Don't take advice from:
there is no difference between a mean well CC/CV power supply and a device sold as a charger. they operate in EXACTLY the same way
Testing has demonstrated that ordinary rim brakes thermally outperformed all but the best disc brakes...You'll always add weight and cost, while not equalling the capabilities of comparable rim brakes, if you use discs

fatty   1 kW

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Location: USA

Re: Super fast charge

Post by fatty » Apr 14 2021 5:10pm

Actually, one way to do this correctly would be to charge through the motor using a suitable controller. Adaptto and Nucular can do this: "Charge mode through the motor with the controller, plug any DC supply to the controller to charge your battery"
Don't take advice from:
there is no difference between a mean well CC/CV power supply and a device sold as a charger. they operate in EXACTLY the same way
Testing has demonstrated that ordinary rim brakes thermally outperformed all but the best disc brakes...You'll always add weight and cost, while not equalling the capabilities of comparable rim brakes, if you use discs

john61ct   100 GW

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Re: Super fast charge

Post by john61ct » Apr 15 2021 9:00am


fatty wrote: I'd expect current limits are deliberately ignored with any charging method.
I'm an advocate of safety, but due to the dynamics of equalization (and because it would be supervised) I actually don't see this as being that dangerous.
Depends on resistance of the source side chemistry.

From Lead to LI no problem, given fat wiring check does not get too hot.

With large capacity LI on both sides and enough voltage differential

even if the wiring does not melt,

too fast an inrush rate will greatly reduce the target pack's cycle lifespan, probably the source as well but not as much if everything is warm.

In cold temps can render instant scrap.

But without knowledge and careful experimenting

the melting wires scenario remains a very real possibility.

john61ct   100 GW

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Re: Super fast charge

Post by john61ct » Apr 15 2021 9:01am

fatty wrote:Actually, one way to do this correctly would be to charge through the motor using a suitable controller. Adaptto and Nucular can do this: "Charge mode through the motor with the controller, plug any DC supply to the controller to charge your battery"
Yes if you can figure it out, that's exactly the type of current limiting needed

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