Max safe Voltage Difference when connecting two Batteries in parallel

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methods   10 GW

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by methods » Nov 12 2019 1:49pm

Your not listening and I dont really like your style flippy.

I am still glad you are here. :D

Try to understand that some of us are talking about setting up tests to debunk incorrect theory. What you are doing right now is being a "Well Meaning Alarmist" with talk of fires.

I am not being condescending when I share a test approach that will prove or disprove a failure mechanism. The disconnect that I highlight by labeling you as "acting like a Vendor" is that I am talking about a Test Setup and you are thinking only in terms of Production Configuration.

Good day to you sir.

-methods
Increasing battery voltage and controller current limit will result in a non linear experience

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by billvon » Nov 12 2019 2:27pm

flippy wrote:
Nov 12 2019 4:51am
using resistors to limit current is bad engeneering and gives unskilled people that read a single topic or posting the idea it might be fine to do.
I'm a power engineer and I use resistors to limit current all the time. It's not bad engineering if you understand what you are doing. M described a method to measure the current you will draw when connecting dissimilar batteries that can be used to design a current limiting resistor that will work.
--bill von

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by flippy » Nov 12 2019 2:57pm

billvon wrote:
Nov 12 2019 2:27pm
flippy wrote:
Nov 12 2019 4:51am
using resistors to limit current is bad engeneering and gives unskilled people that read a single topic or posting the idea it might be fine to do.
I'm a power engineer and I use resistors to limit current all the time. It's not bad engineering if you understand what you are doing. M described a method to measure the current you will draw when connecting dissimilar batteries that can be used to design a current limiting resistor that will work.
i said its -technically- fine, but problem is that most people that read these topics and sometimes even only a single post are NOT in any sense of the word electrically "educated" and will make wrong assumptions. and if you are a enginerd you will know the results from people that -think- they know what they are doing because they read it on a single post of a thread will probably end up burning down their house because of it. people will boil this down to " i need a resistor" and just do whatever and turn their battery wires into filaments and especially if they have pouches probably need to leave the room in a hurry when a pouch starts to pop due to a shorted resistor.

people need to mark posts that "recommend" dangerous stuff with massive disclaimers on public forums, there are way too many dumb people on this planet.
Lithium beats liquid dinosaurs.

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by billvon » Nov 12 2019 6:44pm

flippy wrote:
Nov 12 2019 2:57pm
i said its -technically- fine, but problem is that most people that read these topics and sometimes even only a single post are NOT in any sense of the word electrically "educated" and will make wrong assumptions.
Agreed. But 90% of the posts on this forum are like that. Look at the top non-sticky posts:

Guy wants to build his own distributed 3s6p battery
Guy who did a simple BMS with almost no documentation that will go to 400V (according to him)
Thread about several people who built their own packs out of used cells

Most people in the world, when reading such topics, will either not understand them or understand them so poorly that they will be dangerous. But here, such posts are generally useful.
--bill von

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by john61ct » Nov 12 2019 7:17pm

flippy wrote: if you are a enginerd you will know
I just love that new word so much, hope you don't mind of I borrow it.


> people need to mark posts that "recommend" dangerous stuff with massive disclaimers on public forums, there are way too many dumb people on this planet

I completely disagree, humanity needs more Darwin at work to cut down population numbers, stupidity is an excellent criterion to use.


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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by flippy » Nov 13 2019 1:33am

john61ct wrote:
Nov 12 2019 7:17pm
flippy wrote: if you are a enginerd you will know
I just love that new word so much, hope you don't mind of I borrow it.
> people need to mark posts that "recommend" dangerous stuff with massive disclaimers on public forums, there are way too many dumb people on this planet
I completely disagree, humanity needs more Darwin at work to cut down population numbers, stupidity is an excellent criterion to use.
i fully agree with your sentiment, but usually those dumb people have a tendency to also destroy the lives of others as burning a house down usually also impacts lots of innocent people.
Lithium beats liquid dinosaurs.

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by hias9 » Nov 13 2019 7:58am

It's a bit off-topic, but what I am curious to see would be a diagram how quick-charging affects cycle-life.
Something like the following:
The first graph using normal charging current (for GA: 0.3C), the second using quick charging current (for GA: 0.5C) and the third using an unhealthy quick charging current (for GA: 1C)

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by john61ct » Nov 13 2019 8:40am

At 25°C just accept the maker's datasheet.

Go warm enough, even 1C is no problem at all, maybe even higher.

Go colder and even the lower rates are not optimal for longevity.

But the actual total impact from just this factor is not so great if discharging is already at high rates, avg DoD is high, storing in warm temps and high SoC, etc.

It is just not practical to put hard numbers on so many infinitely variable factors.

If **every** other factor is optimized, maybe a 10% cycle life factor between each of those three C-rates?

Most owners just don't fuss that much.

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by docware » Nov 13 2019 8:51am

hias9 wrote:
Nov 13 2019 7:58am
It's a bit off-topic, but what I am curious to see would be a diagram how quick-charging affects cycle-life.
Something like the following:
The first graph using normal charging current (for GA: 0.3C), the second using quick charging current (for GA: 0.5C) and the third using an unhealthy quick charging current (for GA: 1C)
We have only Pajda´s comments on destructive effect of GA 0,5C charging, no diagrams. If you will go with 1C charging it will be very interesting for ES community if you let us to know next year the impact of such quick charging. :)

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by methods » Nov 13 2019 11:45am

hias9 wrote:
Nov 13 2019 7:58am
It's a bit off-topic, but what I am curious to see would be a diagram how quick-charging affects cycle-life.
Something like the following:
The first graph using normal charging current (for GA: 0.3C), the second using quick charging current (for GA: 0.5C) and the third using an unhealthy quick charging current (for GA: 1C)
Good Question!
At the heart of the topic, not off topic at all.

john61ct wrote:
Nov 13 2019 8:40am
At 25°C just accept the maker's datasheet.

Go warm enough, even 1C is no problem at all, maybe even higher.

Go colder and even the lower rates are not optimal for longevity.

But the actual total impact from just this factor is not so great if discharging is already at high rates, avg DoD is high, storing in warm temps and high SoC, etc.

It is just not practical to put hard numbers on so many infinitely variable factors.

If **every** other factor is optimized, maybe a 10% cycle life factor between each of those three C-rates?

Most owners just don't fuss that much.
Good Answer
I agree that it is more useful to have a "Rule of Thumb" on this (and a layman's understanding) than an Absolute set of numbers or limits. If folks understand the principle of the topic then they can extrapolate their own conclusions.

...

Lithium tolerates being fast charged at Ambient temperature
Lithium does not like being fast charged while cold

Ok -
From that I picture a gooey slurry . . . where some parts are clumpy and other parts are fluid... where when warm it is all more homologous... and if we try to slam current into something clumpy then the current flows in-equally thru the goop leaving some areas more affected than others.

If that were true
Then it would be "hotter is better"
And this may be true, but limited, by some other bounding factors.

....

To flippy's point -

This forum spans everything from the completely incompetent to the epic expert. Part of what we are doing here is bridging that knowledge gap. This is no doubt a place where you can :kff:

... I know Jack about the super intricate details of Lithium Chemistry, but, in trying to cook it down into something a layman can understand ... in making small mistakes... Experts show up, correct (with the details which are hard to comprehend)... then I re-compile and try to cook down again.

That is the age old formula for bridging the gap. Works well. Stoked that everyone is here.

-methods
Increasing battery voltage and controller current limit will result in a non linear experience

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by docware » Nov 13 2019 12:09pm

hias9, do you have any presumption of an average and peak current per cell and average DOD ?

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by hias9 » Nov 13 2019 3:42pm

If you could do such an analysis, that would be really great.
Just use whatever is most comfortable for you as parameters.
If it does not matter, I would say 1C constant discharge and 70% DoD.

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by john61ct » Nov 13 2019 5:03pm


methods wrote:
john61ct wrote:
Nov 13 2019 8:40am
At 25°C just accept the maker's datasheet.

Go warm enough, even 1C is no problem at all, maybe even higher.

Go colder and even the lower rates are not optimal for longevity.
Lithium tolerates being fast charged at Ambient temperature
Well in Alaska at the moment, ambient may be well below freezing, where 0.05C is too fast.

> Then it would be "hotter is better"

For 0.5-7C, I bet human comfort range is fine

For higher then above 30°C

> And this may be true, but limited, by some other bounding factors

For very fast, over 1C, I think 40°C is a reasonable limit, but only for the pre-charging period, while not cycling conditions, cooler the better, so I'm sure trade-offs abound in there, unlikely to get fully quantified.

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by docware » Nov 13 2019 5:33pm

hias9 wrote:
Nov 13 2019 3:42pm
If you could do such an analysis, that would be really great.
Just use whatever is most comfortable for you as parameters.
If it does not matter, I would say 1C constant discharge and 70% DoD.
If I understand properly, both battery packs are finished, correct ?

What is your motor max/average power consumption ?

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by hias9 » Nov 13 2019 6:47pm

My question is very general, not about my specific case.

One battery is finished and will probably be used on two bikes. The faster one will have about 3500W peak (7A per cell), 2000W continous. The slower one only about 1000W peak (2A per cell), about 600W continous.
If 1C charging does not completely destroy cycle life, I could sometimes use my 1800W charger when making a break.

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by sn0wchyld » Nov 14 2019 12:25am

I had to test 8s 28ah batteries for this exact situation in the past - 3 in parallel, fully charged, connected to 1 fully discharged (for worst case scenario). These were not particularly high C rate cells, and were built (series connections) with the expectation of loads no higher than 15A - so pack IR would have been higher than a equivalent sized ebike battery.

Peak current was ~30A, but this dropped rappidly to 15A after about 30 seconds, and trailed off from there over the next 48hrs (~0.3A by that time).

in your case I'd consider the use (for initial testing) of high wattage incandescent light globes (12v 50w versions for example if your delta V is <12-13VDC) as these work as a crude current limiting device, and are easy to hook up in parallel to allow progressively larger currents. Doing so allows you to 'creep up' on the max current that would be achieved if just hooked up straight in parallel. if you graph the results you can extrapolate from a handful of globes (added progressively in parallel, voltage vs total current) to find a good approximation of the 'no globe' parallel current.

There's probably a simple circuit that could be designed to use a globe or similar as a current limiting device, and the short that globe out once the delta V drops below a given value - even a crude analog one could be made using relays or mosfets and a few resistors.

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by john61ct » Nov 14 2019 1:12am

Sounds similar to @method's approach, just using resistors.

Wire length x gauge can do the same to limit current I'm told, fine-stranded boat cable, high temp insulation (silicone?)

Start with a lower V delta, use an infrared thermometer, be conservative about ramping the delta V higher. . .

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by docware » Nov 14 2019 1:41am

hias9 wrote:
Nov 13 2019 6:47pm
My question is very general, not about my specific case.

One battery is finished and will probably be used on two bikes. The faster one will have about 3500W peak (7A per cell), 2000W continous. The slower one only about 1000W peak (2A per cell), about 600W continous.
If 1C charging does not completely destroy cycle life, I could sometimes use my 1800W charger when making a break.
If possible, stop working on the second pack, before you clear up what cells are suitable.
Would be nice if the first battery could be dedicated to the slower bike. You need another charger for this pack, 1C charging is definitely no way for GA. Even 0,5C is too much. GA cells are good for the mild load only, not for high charging and discharging current.
Do you want to run the bikes in the winter ? What temperatures are common there ?

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by sn0wchyld » Nov 14 2019 4:10am

john61ct wrote:
Nov 14 2019 1:12am
Sounds similar to @method's approach, just using resistors.

Wire length x gauge can do the same to limit current I'm told, fine-stranded boat cable, high temp insulation (silicone?)

Start with a lower V delta, use an infrared thermometer, be conservative about ramping the delta V higher. . .
very similar yes, however globes are good because if you happen to hook things up wrong they act as a crude fuse (ie if you dead short the whole pack somehow) - not great as one (globe can explode hahah) but better than a resistor - and their resistance varies according to the current flow - they'll let around 2A through at 2V difference, but only 4A through at 12v difference - an equivalent 12v resistor would only allow about 0.6A through at 2V delta, and would be much larger to dissipate 50W.

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by hias9 » Nov 14 2019 8:22am

docware wrote:
Nov 14 2019 1:41am
If possible, stop working on the second pack, before you clear up what cells are suitable.
Would be nice if the first battery could be dedicated to the slower bike. You need another charger for this pack, 1C charging is definitely no way for GA. Even 0,5C is too much. GA cells are good for the mild load only, not for high charging and discharging current.
Do you want to run the bikes in the winter ? What temperatures are common there ?
I have a 4A charger, but that would be very slow charging. And I am not aware if there is compact 14S 10A or 12A charger.
In the winter I only ride occasionally and short distances. So in the winter I don't need quick charging. Ambient temperatures when I ride are usually 10-30 degrees.
Did you already try what happens to GA cells after about 100 cycles when charging them at 1C at temperatures about 25 degrees?

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by john61ct » Nov 14 2019 9:09am

nominal 35Ah pack

@52Vnominal, 56-57V charging

_______
Grin Satiator is a great little adjustable charger, very safe and reliable, perfect gentle charge for your pack at 8A

Can be stacked when you want to go faster.

About $300

_______
Also to consider, I have dozens of voltage-adjustable Mean Well HLG-240H-54A PSUs in great shape just need wire termination both ends.

each puts out 240W / 4.4A

Also can be paralleled, so 8 of them will get up to your 1C, or scale back in 13% increments as you like.

With these, you need to either end the charge cycle manually, or put in an HV cutoff, maybe driven by a Satiator.

$40 each including conUS shipping, will haggle for 5 or more at a time.



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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by john61ct » Nov 14 2019 9:12am


hias9 wrote:Ambient temperatures when I ride are usually 10-30 degrees.

Did you already try what happens to GA cells after about 100 cycles when charging them at 1C at temperatures about 25 degrees?
° C or F ?

If **F** [edit, corrected]

then you need to make **sure** the inside core of those batteries are warmed up **before you** start charging, bring inside overnight.

Even at very slow rate, but definitely fast, can render useless scrap in one cycle!

By heating to 30° or even higher then fast charging is safer, even 1C might give normal longevity.
Last edited by john61ct on Nov 14 2019 9:12am, edited 1 time in total.

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by hias9 » Nov 14 2019 11:20am

10-30°C (50-86°F)

I think you wanted to say they should be warmed up before charging if it would be 10-30°F which I absolutely agree with.

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by john61ct » Nov 14 2019 1:18pm

Corrected, thanks

At the cold end toward 10°C, should definitely use much slower charge rates than at the warm end.

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Re: Max safe Voltage Difference when connecting two Batteries in parallel

Post by docware » Nov 15 2019 2:15am

hias9 wrote:
Nov 14 2019 8:22am

Did you already try what happens to GA cells after about 100 cycles when charging them at 1C at temperatures about 25 degrees?
No.

Maybe Pajda have such data.

Compact 14s 10 - 12 A chargers exists.

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