a couple of beginner questions

[jesse.rizzo]

Hello all,

Just beginning my DIY powerwall journey and I've got a few unrelated questions.

I've seen in a few places that li-ion batteries have 99% charge efficiency. So if I put in 2000mAh into a battery, safe to say if has pretty close to 2000mAh capacity. Right? Any reason it's better to test capacity during discharge than charging?

You're supposed to cut of charge when the charge current drops to 3% of the capacity. Is that 3% of the original capacity, or the actual capacity. Ie if a 2000mAh battery only has 1000mAh of capacity left, do we cut off charge at 60 or 30 mA?

Is a BMS necessary if I'm willing to babysit my batteries. It will only be for emergencies, so I'll probably only need it for a day. Then I can disconnect everything and just charge in parallel. If I do this can I go without a BMS?

Thanks for any help.

 

[DarkRaven]

I've seen in a few places that li-ion batteries have 99% charge efficiency. So if I put in 2000mAh into a battery, safe to say if has pretty close to 2000mAh capacity. Right? Any reason it's better to test capacity during discharge than charging?

I don't know if it is 99%, but yes, it is very high. However, it depends on how you discharge the cell what the actual usuable capacity is, so testing capacity during discharge is still the more accurate method. And also, to fully charge the cell to measure capacity it has to be fully discharged first, so you can always do both anyway if you want.

You're supposed to cut of charge when the charge current drops to 3% of the capacity. Is that 3% of the original capacity, or the actual capacity. Ie if a 2000mAh battery only has 1000mAh of capacity left, do we cut off charge at 60 or 30 mA?

It is not always 3% and not related to the capacity as such, but the initial charging current. That in turn might be related to the capacity. It depends on the cell though. In its datasheet the manufacturer will have written down what the cutoff current is.

Is a BMS necessary if I'm willing to babysit my batteries. It will only be for emergencies, so I'll probably only need it for a day. Then I can disconnect everything and just charge in parallel. If I do this can I go without a BMS?

A BMS will provide several functions, depending on what type of BMS you use. It protects the cells from overcharge and overdischarge, it keeps them balanced, it watches for the temperature and so on and so forth.
If you NEED one or SHOULD USE one (two entirely different things) depends on how you use the battery. If you want your powerwall to be fully autonomous, then you need one. If you want it to run unattended you probably should use one. However if you have a close look at voltages, currents, state of charge and so on during operation then you can probably use your system without a BMS. You should still look into battery safety though, monitoring the system manually and not using it when you are away doesn't mean you shouldn't use the usual safety precautions.

 

[jesse.rizzo]

I don't know if it is 99%, but yes, it is very high. However, it depends on how you discharge the cell what the actual usuable capacity is, so testing capacity during discharge is still the more accurate method. And also, to fully charge the cell to measure capacity it has to be fully discharged first, so you can always do both anyway if you want.

Makes sense. I'm building my own Arduino charger, and I'm an Arduino newby, so I'm looking for ways to make it simpler. I'll probably just measure charging Ah for now and add discharge testing later.

It is not always 3% and not related to the capacity as such, but the initial charging current. That in turn might be related to the capacity. It depends on the cell though. In its datasheet the manufacturer will have written down what the cutoff current is.

The batteries I'm playing around with now, the datasheet says cutoff is 7 hours or 0.05C. But that still raises the question, is C based on the original capacity, or the capacity they have now?

A BMS will provide several functions, depending on what type of BMS you use. It protects the cells from overcharge and overdischarge, it keeps them balanced, it watches for the temperature and so on and so forth.
If you NEED one or SHOULD USE one (two entirely different things) depends on how you use the battery. If you want your powerwall to be fully autonomous, then you need one. If you want it to run unattended you probably should use one. However if you have a close look at voltages, currents, state of charge and so on during operation then you can probably use your system without a BMS. You should still look into battery safety though, monitoring the system manually and not using it when you are away doesn't mean you shouldn't use the usual safety precautions.

I don't expect it to be fully autonomous for what I'm planning on spending. I'm not too worried about balancing. So I'll probably just rig up a temp monitor and undervoltage cutoff.

 

[BlueSwordM]

When working with lithium-ion cells, specifically 3.7V 18650 cells, you want a conservative cutoff voltage like 3.2-3.4V rather than remaining capacity, as 95% of the capacity is between 3.2V and 4.2V.

So when discharging, set a cutoff voltage of 3.2V for capacity testing, and 4.1V charging voltage, as charging to 90% capacity can double, or even triple cycle life of your batteries.
Also, couldn't you just use a 4.2V/8.4V/12.6V/etc power supply for charging, and the Arduino for monitoring and bottom balancing(charging cells at the bottom)?

 

[daromer]

Dont make it too complicated i would say. Top balance each time you charge them. Alot easier than bottom balancing packs unless you want to drain them..
Make the packs even and they will be even during discharge.

Test the full range and just make it simple. If you want the see how they perform in end you test the capacity when they pack is built!
You can ofcourse run without a BMS..: Though battery voltage monitoring is one part of a BMS...

All RC stuff uses simple "BMS".... They only monitor voltage so you know when to land and then you balance charge it til top balance and full. The same pricniple can be used on all devices where you charge and use once...

 

[DarkRaven]

If it says 0.05C in the datasheet then this is in relation to the rated capacity. This doesn't change with the actual reimaining capacity. However this is just important for the manufacturer to provide information to the customer on how the specification for this cell was done. You can alsway cutoff earlier if you want. Or later.

If you cut off early you might not have the full capacity, if you cut off late you might be wasting time for almost no additional capacity.

 

[jesse.rizzo]

So when discharging, set a cutoff voltage of 3.2V for capacity testing, and 4.1V charging voltage, as charging to 90% capacity can double, or even triple cycle life of your batteries.
Also, couldn't you just use a 4.2V/8.4V/12.6V/etc power supply for charging, and the Arduino for monitoring and bottom balancing(charging cells at the bottom)?

I was planning to cycle them between 3 and 4 volts or so. And I am using a CC-CV buck converter for charging and the Arduino to sense current into the battery and cut it off when the current drops low enough.

Dont make it too complicated i would say. Top balance each time you charge them. Alot easier than bottom balancing packs unless you want to drain them..
Make the packs even and they will be even during discharge.

Test the full range and just make it simple. If you want the see how they perform in end you test the capacity when they pack is built!
You can ofcourse run without a BMS..: Though battery voltage monitoring is one part of a BMS...

All RC stuff uses simple "BMS".... They only monitor voltage so you know when to land and then you balance charge it til top balance and full. The same pricniple can be used on all devices where you charge and use once...

I was planning to just charge in parallel, but I just watched a youtube video on how parallel charging doesn't replace the need for balancing. So I'll probably top balance since this pack will spend a lot more time close to fully charged than to discharged.

If it says 0.05C in the datasheet then this is in relation to the rated capacity. This doesn't change with the actual reimaining capacity. However this is just important for the manufacturer to provide information to the customer on how the specification for this cell was done. You can alsway cutoff earlier if you want. Or later.

If you cut off early you might not have the full capacity, if you cut off late you might be wasting time for almost no additional capacity.

Thank you, I couldn't find that anywhere. I know I can cut off earlier if I want to, but it's good to know the limit.

 

[jesse.rizzo]

Ok, here's another one. I'm looking for a more modular way to build packs. Seems the standard way is soldering nickle strips. But I'm probably going to change things a lot before I settle on a final configuration, so if there's a way that wouldn't require soldering and desoldering dozens or hundreds of wires every time that would be ideal.

 

[DarkRaven]

Nickel strips are spotwelded, not soldered. Soldering is done with copper busbars and fusewires instead. For smaller tests you can use these 4 piece 18650 holders, for bigger setups there is no known working and cheap system to make a modular pack.

 

[SimonW]

You could use cell holders like Darmok has done here :http://secondlifestorage.com/t-Camping-24p4s-16-8v-pack

 

[jesse.rizzo]

Nickel strips are spotwelded, not soldered. Soldering is done with copper busbars and fusewires instead. For smaller tests you can use these 4 piece 18650 holders, for bigger setups there is no known working and cheap system to make a modular pack.

Good to know, is there any advantage to nickle vs copper?What gauge are people using for fusewires? Yeah, I was afraid there would be no easy way to do it.

You could use cell holders like Darmok has done here :http://secondlifestorage.com/t-Camping-24p4s-16-8v-pack

I'd looked at those, unfortunately they are like 4 times as expensive per cell as those 4 x 5 grid holders.

 

[DarkRaven]

Nickel is about four times as resistive as copper as an electrical conductor, but still very good. At the same time it is about four times easier to use when spotwelding a pack Spotwelding is faster than soldering as well.
The size of your fusewire depends on your intended load. You need to know your current draw first and then you can size the fusewire. It needs to be big enough to withstand the current during usual operation but small enough to fuse under heavy loads, like massive overload due to short circuit.

 

[daromer]

Im using 35AWG = 0.2mm and it works ok. 5-7A roughly. Tinned copper fuse wire

 

[sunseeker]

I will also start using tinned or silver coated 0,2mm copper wire. I used plastic coated copper wire until now, but getting rid of the coating is always a big pain in the neck.
I can only find silver coated copper wire, but I guess the ampere rating will be in the same ballpark.

Which thickness do you use for soldering the minus side?

 

[jesse.rizzo]

Is there anything special about tinned copper, or can I just use 35awg regular copper wire?Alternatively, could I just pull a few strands out of a thicker gauge stranded wire and use that?

 

[DarkRaven]

Is there anything special about tinned copper, or can I just use 35awg regular copper wire?

There are usually two reasons why you would tin something. First is soldering, it is easier so solder pre-tinned metals. Second is protection, tin helps to prevent corrosion/oxidation.

Alternatively, could I just pull a few strands out of a thicker gauge stranded wire and use that?

Sure, you could totally do this.

 

[SimonW]

...Alternatively, could I just pull a few strands out of a thicker gauge stranded wire and use that?

That is exactly what I have done.
I bought 1 metre of general hookup wire from an electronics retailer (Jaycar here in Australia)https://www.jaycar.com.au/red-flexible-light-duty-hook-up-wire/p/WH3010

This has 13strands of 0.12mm (0.0113mm soapprox.36AWG)

13 metres (43 feet) of ~36 AWG tinned. I'll never use it all.
And it cost25 cents....