building my first battery

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parrst

1 mW
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Mar 23, 2020
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First off, I must admit I am not making a bike, rather a battery system for a ham radio portable kit. I am using LEFPO4 batteries at 15AH each. I am building an 8 cell $S battery. I have two questions.

First question; is it best to build the battery with 4 sets (in series) of 2 cells (in parallel) and use one BMS, or make to 4 cell 4S batteries, each with it's own BMS and put them in parallel. I am assuming the first option as I have seen many videos done this way and non doing it the second way.

Second question; I got each set of 4 cells from two places. both have cells that stay pretty close on the voltage (within about .15V difference max). I tested both in a 4 cell 4S configuration partly due to them arriving at different times and partly due to the time to charge/discharge 30AH vs 15AH. One set has just over 16AH charging to 14.6v and discharging to 12v. The other set has closer to 15AH ( forgot to write down on the charge cycle, but discharged to 14.8AH) charging to 14.8V and discharging to 12V. Is that enough difference to worry? And should I put one cell from each batch in parallel, or keep the two batches separate?

Thanks for any help,
Bob
 
I would go with the 4s2p battery pack 30ah pack. For the most accurate pack mah you would have to capacity test every cell and then use repackr to tell you where to put each cell.

Since with 15ah cells it would take along time to test each cell, as long you have them pretty closed on the mah, just put them together. Once put together the voltage will equalize, If there is any difference it will be small. Instead of a 30ah pack, you might have a 29ah pack. Just make sure none of the cells self discharge before building the pack, which will cause balancing problems.

With a 4s lifepo4 you don't want to charge past 14.6 volts. (3.65 per cell)
 
Proper label is LiFePO4, or LFP.

I am used to dealing with 400-1200Ah banks, so this is tiny. Are these cells prismatics, Headways or what?

Where the cells secondhand, or cheap, under say $50 each?

And how much trouble & expense do you want to invest to "do it right" shoot for over a decade and thousands of cycles?

Usually you'd want to pair up in parallel first, then connect those pairs / groups into the 4S string, aka 2P4S.

But I'd only do that with new / quality cells proven to be well matched, really should test out identical both capacity (CC load testing) and ESIR resistance.

If the two sets turn out to not match closely, might be best to build two separate packs 1P4S and then use those in parallel.

To start with yes all can go in one group for manual balancing, if at the top, charge to full, say 3.6V is plenty, then sit paralleled for say 48hrs. Then atomize sit 24hrs isolated and see if any drop.

For normal usage, best for longevity to stay below 13.85V, 3.45Vpc if using per-cell HVC balance charging.

Each sub-pack should have its own little BMS so you can track per-cell voltages, ideally cut the load based on per-cell LVC, well above 3.0Vpc (12V)

Unless you're just keeping a close eye manually, but if new / expensive cells I would not risk it.
 
4 are headway the other 4 were supposed to be, but I think they may be knockoffs or "comparable", they are the slightly less capacity cells. I plan to use a solar charger/controller so that I can cut the use at 12 volts. Most of the BMSs I found have a cuttoff at 2V +-.1. That is WAY lower than I want to go. But I figure if the pack cuts off at 12, I should be okay.

Cells were about $25 each or so as I recall.

"But I'd only do that with new / quality cells proven to be well matched, really should test out identical both capacity (CC load testing) and ESIR resistance.

If the two sets turn out to not match closely, might be best to build two separate packs 1P4S and then use those in parallel.

To start with yes all can go in one group for manual balancing, if at the top, charge to full, say 3.6V is plenty, then sit paralleled for say 48hrs. Then atomize sit 24hrs isolated and see if any drop."

I think my battery charger can tell me resistance of the cells and what exactly is CC load testing. My charger will discharge at roughly 20W so with the full 4S mode only about 1.1 amps I am pretty sure it does CC all the way to the set voltage. So I can take the time to test each cell, but I don't plan on throwing any of these away unless really bad.

What does atomize? is that just braking the cells back apart? And how far from identical is too far?

Bob
 
Link to the controller please, and explain how you can set it up to act as an LVC.

Any decent BMS has adjustable setpoints.
 
John, where do I find a decent BMS? My max current draw is only going to be 20 or 30 amps at most, and most of the time much lower, 5 or so jumping to 10 - 18 at most when I transmit and THAT is if I use for my big radio. I do also plan to use this for my CPAP when I camp and it can draw 3 - 3.5 amps or so and I sleep 8 hours when camping, 10 if I am really tired and it doesn't get too hot. lol

https://www.amazon.com/gp/product/B07H86ZP6C/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1
you can program a high and low voltage cutoff

is charge controller but I also have an i Charger 106B+ for charging at up to 7 amps CC. I plan to do the 30AH pack at about 6 most of the time. I was told charge between 20 and 50% C.
 
There are hundreds of BMS, so many variables especially longevity / reliability, I just don't reco them.

jonyjoe303 will reco the Chargery he uses, not expensive.

If you do use a BMS for LVC, then you won't need to do whatever you were thinking with the SC, personally can't see how that would work.

0.4C is a decent max charge rate for say 20°C and warmer, as it gets cooler go slower, and not at all below freezing.
 
john61ct said:
There are hundreds of BMS, so many variables especially longevity / reliability, I just don't reco them.

jonyjoe303 will reco the Chargery he uses, not expensive.

If you do use a BMS for LVC, then you won't need to do whatever you were thinking with the SC, personally can't see how that would work.

0.4C is a decent max charge rate for say 20°C and warmer, as it gets cooler go slower, and not at all below freezing.
Click to expand...

The solar charge controller will cut off charging current if the batter gets to a certain voltage, and cut off the load if the battery voltage gets to low

I did try to find a lvc since most of the BMSs I could find had way too low a cutoff voltage, but most seemed to be expensive and for much more current than I need.

Bob
 
You can put two sightly different capacity cells in parallel they will act as one cell. The series set should all have same capacity if possible. So if you test all cells and write down info on each cell. Number them. So after finding out capacity match the headways and no name and match capacity.. A BMS is a must as headways can need balancing attention.. You can put an extra set of 4s sense wire to help to check and balance.ez. And with screw tops you adjust things later. P S. Do not over tighten
How much run time will you need.
 
999zip999 said:
You can put two sightly different capacity cells in parallel they will act as one cell. The series set should all have same capacity if possible. So if you test all cells and write down info on each cell. Number them. So after finding out capacity match the headways and no name and match capacity.. A BMS is a must as headways can need balancing attention.. You can put an extra set of 4s sense wire to help to check and balance.ez. And with screw tops you adjust things later. P S. Do not over tighten
How much run time will you need.
Click to expand...

Thank you. THIS is what I needed most. But I do appreciate all the advise. Unfortunately the BMSs I can find mostly cut off way too low, and I had a hard time finding a voltage cut off at the point I wanted that wasn't way over kill, but I have a charge controller that will do what I want.

Bob
 
If you're talking about running all your loads through a SC, again good luck.

Most are pretty fussy about the type of load they will support

and are rated at very low power.

Just use an adjustable BMS of enough ampacity, or even a standalone LVC.

A SC is for charging from panels.
 
Do balance the parallel groups to same voltage before assembly or before installing BMS. The bms will help keep a balance pack balance. I have three voltpheaks lifepo4 single cell charger @ 3.6v at 2amp. I haven't seen them in a while but someone must sell something like.
Get a piece of paper and write it down number your cells or give them names .As you are now married.
 
john61ct said:
If you're talking about running all your loads through a SC, again good luck.

Most are pretty fussy about the type of load they will support

and are rated at very low power.

Just use an adjustable BMS of enough ampacity, or even a standalone LVC.

A SC is for charging from panels.
Click to expand...
My use is pretty low power compared to an electric vehicle. probably 20 amps at most, and more likely closer to 10 at transmit. Also for an adjustable BMS or LVC, a source or brand/model would be helpful

Bob
 
OK, but what is the ampacity limit for the load terminals of your SC?

And do they state that all load types (e.g. resistive vs inductive) are supported?

______
people think "a BMS" should be available as a single product.

Best IMO to look at it as functionality, shared across simple robust OTS devices.

If live cell balancing is removed from the req list, much less prone to failure.

That leaves LVC, HVC and maybe temperature protection.

Implementation at pack- or bank- level depends on the use case.

Redundancy required for mission-critical or very large expensive banks.

All in addition to charge control and monitoring by the user, which can in fact make many automated systems unnecessary, substitute time and diligence for capital investment.

______
As stated for "a BMS" brand, Chargery is one joey and others reco. Daly is also frequently mentioned, as used by many members as well

Otherwise, google or forum search for "bluetooth BMS".

There are many dozens of threads on just this topic

e.g. https://endless-sphere.com/forums/viewtopic.php?t=104246
 
999zip999 said:
Do balance the parallel groups to same voltage before assembly or before installing BMS. The bms will help keep a balance pack balance. I have three voltpheaks lifepo4 single cell charger @ 3.6v at 2amp. I haven't seen them in a while but someone must sell something like.
Get a piece of paper and write it down number your cells or give them names .As you are now married.
Click to expand...

If using a BMS I can charge with my hobby charger at up to 7 amps constant current then switches to CV till current is 1/10 the charge current. I can also bypass the BMS and use an adapter (at least that is how I plan to wire it up) to charge with that same charger using the chargers' balancing.
 
Bed-Electronics sells LVC/HVC, including on eBay.

Lots of cheap-Chinese google for phrases like "Board Under Voltage Over Voltage Protection Module" most have pretty good adjustable ranges.

Obviously a decent charger is giving one layer of defence for HVC, another is just for redundancy withsuch cheap cells most don't bother.

For LVC google for "battery protect" devices.

Balancing is a whole different topic, also can be well handled by a good adjustable charger.
 
john61ct said:
OK, but what is the ampacity limit for the load terminals of your SC?

And do they state that all load types (e.g. resistive vs inductive) are supported?

Best IMO to look at it as functionality, shared across simple robust OTS devices.

As stated for "a BMS" brand, Chargery is one joey and others reco. Daly is also frequently mentioned, as used by many members as well

Otherwise, google or forum search for "bluetooth BMS".

There are many dozens of threads on just this topic

e.g. https://endless-sphere.com/forums/viewtopic.php?t=104246
Click to expand...

What is ampacity? current capacity? My pack will be 30 AH

As for the brands, lol my bad, I thought chargery, I thougth that was a misspeling, joey was another user in the group and reco was short for recomend. lol Thank you for clearing that up.

My investment is not huge and I want to keep it that way. I know everything is a trade off, capacity vs battery life and so on. I am looking for much of it to be automatic or at least have a fail safe so I don't space and kill the batteries.

Bob
 
No CV stage at all is best for longevity.

Just stop at 3.45-3.5Vpc

A balancing device that forces you to sit at Full for more than a few minutes should be avoided.

If balancing is only needed say quarterly or even better annually, then NBD, depends on how healthy and matched the cells are.

Be sure to regularly test & balance the individual cells, not just whole groups at a time.
 
Figure $30 per Ah @12V overall cost for a good LFP system based on new quality cells, pretty cheap if it lasts over a decade.

The cheaper the bank the more you need better infrastructure to automate its protection.

so if you only spend $10 per Ah on the cells, might need to spend double that for the rest to get good security.

Or just take a chance on having to buy a replacement bank after a couple/few years, as is normal for lead.
 
I had not raised the issue of the wiring.

>> what is the ampacity limit for the load terminals of your SC?
 
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