270ah DIY LiFePO4 build

[Dogone]

It used to be recommended, Is it a bad idea to balance overnight in parallel?

 

[Poey50]

It used to be recommended, Is it a bad idea to balance overnight in parallel?

If you follow the downloaded instructions to the letter in setting the voltage of the PSU before connecting to the paralleled cells then in theory you can't overcharge. However, given so much is at stake, I checked the voltages of all cells every twenty minutes. When I needed to sleep I stopped the charging and just left them connected in parallel with no charge. This does nothing but does no harm either. The parallel stage of charging (following the charging of the 4S pack at 12 volts) took 8 hours.

 

[Dogone]

If you follow the downloaded instructions to the letter in setting the voltage of the PSU before connecting to the paralleled cells then in theory you can't overcharge. However, given so much is at stake, I checked the voltages of all cells every twenty minutes. When I needed to sleep I stopped the charging and just left them connected in parallel with no charge. This does nothing but does no harm either. The parallel stage of charging (following the charging of the 4S pack at 12 volts) took 8 hours.

Sorry, I wasn’t clear enough, I meant to say the way previously recommended (actually by Collins) was to charge up in series, to disconnect and then re-wire all cells in parallel and then let them sit overnight or even better, for a couple of days with no charger connected so they could balance themselves. Left like this there will be a small current flow from the slightly higher voltage cell to the lower voltage ones.

Is that a bad idea now?

 

[Poey50]

Sorry, I wasn’t clear enough, I meant to say the way previously recommended (actually by Collins) was to charge up in series, to disconnect and then re-wire all cells in parallel and then let them sit overnight or even better, for a couple of days with no charger connected so they could balance themselves. Left like this there will be a small current flow from the slightly higher voltage cell to the lower voltage ones.

Is that a bad idea now?

Are you sure that was Collins? He does recommend a different method which is to top balance progressively in parallel to 3.4v, 3.5v and then 3.6v. That doesn't seem to confer any additional benefit to the method I linked to and used.

It is generally reckoned that cells don't balance in parallel unless there is a charge running through them so I don't recognise leaving them uncharged overnight or longer as standard good practise. Of course thinking has changed a fair bit in the last few years - not long ago balancing to a higher voltage - e.g. 3.8v per cell was common. No-one would go so high now.

 

[nfluester]

So I finished top balancing mine using a hobby charger as i have loads of them as i fly RC aeroplanes and helicopters (when not on the boat) i set the LIFE end voltage to 3.6v and wired as an 8p1s cell (2240AH ?) and left it running, it took ages so reconfigured as a 2 x 4s1p and used 2 chargers and got them both up close and then reconfigured back to parallel to finish, did a capacity test yesterday using a 1500W fan heater through an inverter drawing 123 amps just editing the video up and will upload hopefully tomorrow ?

 

[Dogone]

Are you sure that was Collins? He does recommend a different method which is to top balance progressively in parallel to 3.4v, 3.5v and then 3.6v. That doesn't seem to confer any additional benefit to the method I linked to and used.

It is generally reckoned that cells don't balance in parallel unless there is a charge running through them so I don't recognise leaving them uncharged overnight or longer as standard good practise. Of course thinking has changed a fair bit in the last few years - not long ago balancing to a higher voltage - e.g. 3.8v per cell was common. No-one would go so high now.

Quite sure, especially now I just checked my not so bad memory.

He also said to top up the balance to 3.8v with a bench top supply after letting it sit for a day or two.

See:
LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks - Page 215 - Cruisers & Sailing Forums

 

[nfluester]

here are a few pics



the highly dubious test setup



after 4 hours at around 132 Amps had drawn 511AH



you'll have to watch the video when i post it to find out what happened after that.......

 

[Poey50]

Quite sure, especially now I just checked my not so bad memory.

He also said to top up the balance to 3.8v with a bench top supply after letting it sit for a day or two.

See:
LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks - Page 215 - Cruisers & Sailing Forums

Very interesting, thanks. Shows how the ideas have changed in 7 years. This is what he writes now in his Marine How To article ...

"Balancing Via Parallel Resting Voltages???

Many often assume that by simply wiring the cells in parallel they will magically get themselves in balance. This is not entirely true, if you expect it to happen in a timely manner. When cells are wired in parallel, the the cell voltages attain a parity voltage rather quickly. Once a parity voltage is attained the transfer or movement of current between cells, in order to balance SoC, slows to a crawl. Ohms law is in control here and we are talking 0.0001A level movements of current. Attaining a true balancing, by letting cells sit in parallel, at a resting non-charging voltage, takes a very long time. You can let them sit for a week or more, but again, this may not be enough time. Balancing ideally requires a voltage differential to move current between or into the cells. When cells are at the same voltage this transfer of current = slow.

You can drastically speed the process by presenting the parallel wired cells with a charging voltage.. The PSMTB method is the fasted way we know of to attain a perfect balance. Once all cells are at the same voltage and no more current can flow into the cells they are all at the identical SoC."

 

[Poey50]

The ABYC have very recently issued technical guidance for LFP batteries (TE-13). This takes on the problem of non-communicating drop-in LFP batteries by requiring that the BMS sound an alarm prior to a disconnect (for any reason). Although not yet an enforceable requirement in the US these technical directions are, I gather, used as benchmarks by surveyors and, by extension, insurers so that might over time help to create a small market where drop-ins more suitable for marine use might emerge.

 

[Kelpie]

I'm still trying to get to grips with the basics of this technology, so forgive me...
If the BMS does not communicate, this is bad because when it disconnectes
a) you could fry your alternator
b) you could, without warning, lose your power source to nav instruments etc

Are those the only considerations, or is it more complicated than that? (not trying to downplay that the above reasons are serious, just considering ways to work around them).

 

[vas]

I'm still trying to get to grips with the basics of this technology, so forgive me...
If the BMS does not communicate, this is bad because when it disconnectes
a) you could fry your alternator
b) you could, without warning, lose your power source to nav instruments etc

Are those the only considerations, or is it more complicated than that? (not trying to downplay that the above reasons are serious, just considering ways to work around them).

c) kill the LifePO4 pack as well I guess

 

[Kelpie]

c) kill the LifePO4 pack as well I guess

Again excuse my ignorance, but why would this happen? Is it because a cheap BMS will only cut in when the battery is dangerously out of line? I see a lot of them don't disconnect until 10v, which sounds far too low for a happy and long lived battery.

 

[Poey50]

I'm still trying to get to grips with the basics of this technology, so forgive me...
If the BMS does not communicate, this is bad because when it disconnectes
a) you could fry your alternator
b) you could, without warning, lose your power source to nav instruments etc

Are those the only considerations, or is it more complicated than that? (not trying to downplay that the above reasons are serious, just considering ways to work around them).

The BMS in a drop-in has one job only which is to protect the LiFePO4 from charge, discharge, low temperature, and high temperature parameters being exceeded. And it has one method to achieve this which is to disconnect itself from your electrical system.

The worst case scenario is if you are motoring and charging with the alternator and there is a disconnect through the high voltage parameter being exceeded. This a) leaves you with no power so all lights, nav systems, VHF etc are lost, b) creates a high voltage spike that takes out the alternator diodes, and c) the same high voltage spike can fry expensive marine electronics.

There are a variety of work-arounds for all these but this is the first question to ask when installing LFP: how do I manage a disconnect?

 

[Poey50]

c) kill the LifePO4 pack as well I guess

Well, no. That is the problem. The only thing the BMS of a drop-in will do is to protect the LFP and itself. It's like 'America-First' for batteries. Some BMSs do however have parameters set too low or too high for long life (to compensate for poor charging practices) so if sudden disconnect happens repeatedly then you will lose out on the investment but, if happening on a boat, you will probably have bigger problems before that happens.

 

[Poey50]

Again excuse my ignorance, but why would this happen? Is it because a cheap BMS will only cut in when the battery is dangerously out of line? I see a lot of them don't disconnect until 10v, which sounds far too low for a happy and long lived battery.

Yes 10v is very low for disconnect. This BTW is one of the cautionary factors to be kept in mind in considering the Zwerfcat hybrid system. That involves using the BMS to routinely take the LFP on and off line. That needs some conservative settings for the high and low voltage disconnect parameters (max 3.65v per cell and min 2.9v). Those wouldn't be found in some of the cheaper drop-ins. In fact Zwerfcat use their own external configurable BMS. It would also be achievable with the 123SmartBMS.

 

[Kelpie]

Yes 10v is very low for disconnect. This BTW is one of the cautionary factors to be kept in mind in considering the Zwerfcat hybrid system. That involves using the BMS to routinely take the BMS on and off line. That needs some conservative settings for the high and low voltage disconnect parameters. Those wouldn't be found in some of the cheaper drop-ins. In fact Zwerfcat use their own external configurable BMS. It would also be achievable with the 123SmartBMS.

Thanks. Every day is a school day. Most of the really cheap drop-ins have a 10v cutoff, and I presume that can't be changed. I presume that the only way you can really use these batteries in practice is to manually monitor the voltage and disconnect. But I suppose I do that already with my lead-acids.

Is there an idiots' guide for BMS anywhere?

 

[Poey50]

Is there an idiots' guide for BMS anywhere?

Spend time on the DIY Solar site. There is a lot of discussion about low cost BMSs there. But no simple guide to my knowledge.

 

[nfluester]

So I have managed to do a rather sketchy preliminary capacity check if you want to follow along with the build where i pulled 130amps for about 4 and a half hours

 

[gregcope]

@nfluester What BMS/Shunt is the one you are using on that video?

 

[nfluester]

Here is a link to the BMS US $7.02 10% OFF|3S 12.6V or 4S Lifepo4 or Lithium Ion Battery smart Bluetooth BMS 12V with 100A or 150A constant current UART communication|Integrated Circuits| - AliExpress

Seems to work OK and has a Bluetooth app to view everything that's going on