diy solar

diy solar

Can I pre-charge before a top balance with my hybrid inverter?

Devo1982

New Member
Joined
Apr 11, 2023
Messages
26
Location
Occidental Mindoro, Philippines
Hi all.

Newbie but very keen learner here.

I have 8 x 230ah lifepo4 cells on their way to me at the moment. I have read the excellent top balancing pdf tutorial and was wondering if I can use my 3kw hybrid solar inverter to initially give the cells a decent charge in series with a BMS before paralleling them and using a PSU to do the actual top balance? My hybrid inverter is currently set to charge at 20 amps but that can be adjusted in increments of 10 if I wanted to give them more initially.

TIA
 
Last edited:
initially give the cells a decent charge in series with a BMS before paralleling them
Absolutely, its a fairly common way to get a head start on top balancing. At 3.2V, its a LOT of amp hours to charge from empty in parallel.
Be sure to set voltage BEFORE connecting to battery.

Label your cells and keep track of which cell(s) top out first. These are likely your lower capacity cells. Good notes from the start helps a lot down the road.
 
Absolutely, its a fairly common way to get a head start on top balancing. At 3.2V, its a LOT of amp hours to charge from empty in parallel.
Be sure to set voltage BEFORE connecting to battery.

Label your cells and keep track of which cell(s) top out first. These are likely your lower capacity cells. Good notes from the start helps a lot down the road.
Thanks for the reply!

Will it be safe to set my inverter's bulk charging voltage to, say, 28.8v for the initial pre-charge? How many amps would be safe? .2c? And I'm guessing I'll know when the cells are all pretty charged as amps on inverter display will drop to zero?


IMG_20230412_083101.jpg
 
say, 28.8v for the initial pre-charge? How many amps would be safe? .2c?
28.8V / 4 = 3.6Vpc

Its unlikely that your charging will get to this voltage for 8 cells without the BMS hitting cell over volt protection.

The faster you charge, the less likely they will charge evenly.
 
So what would you recommend please?
Since these are new cells, i'd try to learn as much as possible early on. Charge to 13.8V and see if they are still close in voltage (all below 3.55V (good), all below 3.50V (very good). Take notes as to which are the runners.
Then up to 14.0V. If you can hold them here with constant voltage for an hour or 2, they will absorb evenly and be pretty much fully charged. Safely. From here a top balance would be fairly quick.

If you have the time to charge slower, like .1C, then that will put on a more even charge. If you are in a hurry (its really easy to be impatient with this slow process!), .2C would be fine. Not sure it will save any time when you need to deal with more imbalance issues.
 
Absolutely, its a fairly common way to get a head start on top balancing. At 3.2V, its a LOT of amp hours to charge from empty in parallel.
Be sure to set voltage BEFORE connecting to battery.

Label your cells and keep track of which cell(s) top out first. These are likely your lower capacity cells. Good notes from the start helps a lot down the road.
Mister Sandals,

the ones that top up first would that be due to a lower internal resistance? and if so would that not indicate that they are healthier than one with more internal resistance? (caveat: i have no clue just taking a guess.)
 
If your BMS has an active balance built in, the battery may not need to be set back in parallel for top balance.

28.8 charging is fine. Might just make it. And if all is good to 28.8 I would put direct in service and skip the top balance.
 
Mister Sandals,

the ones that top up first would that be due to a lower internal resistance? and if so would that not indicate that they are healthier than one with more internal resistance? (caveat: i have no clue just taking a guess.)
The “runners” on charge are usually the weakest, however you can’t judge them on the first charge. Get them to top evenly and check again after another full charge and chances are it’s all good.
 
I have been running my banks for over 18 months now and am just curious for learning sakes...
I have two S16 banks that sit at .005 or less divergence, a thrid bank that when charged past 54 ends up with a .035 spread but once charging is done and there is any draw it quickly drops down to .005~.007
The fourth bank when charged past 54 gets as bad with .060 spread and once charging is done and their is any load it drops down to .023~.026

the last two packs might not have been properly top balanced (they were added 3 months after the first two.) as my power source was acting funny and I was in a bit of a hurry due to the onset of winter. the spread increased over the last 14 months, and with weeather and work I did not have time to muck with them.

I just pulled the fourth bank and am busy top balancing them for the second time with a new power supply to see if that was actually the issue. once it is done i will re-install and top balance bank #3 the same way and see how the both of them act.
 
I have been running my banks for over 18 months now and am just curious for learning sakes...
I have two S16 banks that sit at .005 or less divergence, a thrid bank that when charged past 54 ends up with a .035 spread but once charging is done and there is any draw it quickly drops down to .005~.007
The fourth bank when charged past 54 gets as bad with .060 spread and once charging is done and their is any load it drops down to .023~.026

the last two packs might not have been properly top balanced (they were added 3 months after the first two.) as my power source was acting funny and I was in a bit of a hurry due to the onset of winter. the spread increased over the last 14 months, and with weeather and work I did not have time to muck with them.

I just pulled the fourth bank and am busy top balancing them for the second time with a new power supply to see if that was actually the issue. once it is done i will re-install and top balance bank #3 the same way and see how the both of them act.
Have you considered an active balancer or to use BMS balancing?

I had some divergence in cells and changed my balancer settings from “above 3.4 when charging” to “Whenever above 3.4” and within 5 days, the voltages grew closer. Cells spent so little time above 3.4 while charging that there was very little time spent in equalization.

That worked for me.

Amazon has 16s active equalizers for between $55 and $75.
 
Since these are new cells, i'd try to learn as much as possible early on. Charge to 13.8V and see if they are still close in voltage (all below 3.55V (good), all below 3.50V (very good). Take notes as to which are the runners.
Then up to 14.0V. If you can hold them here with constant voltage for an hour or 2, they will absorb evenly and be pretty much fully charged. Safely. From here a top balance would be fairly quick.

If you have the time to charge slower, like .1C, then that will put on a more even charge. If you are in a hurry (its really easy to be impatient with this slow process!), .2C would be fine. Not sure it will save any time when you need to deal with more imbalance issues.

Seeing as I'll be configuring eight of them in series, I assume I can double the voltages you've suggested?
 
Have you considered an active balancer or to use BMS balancing?

I had some divergence in cells and changed my balancer settings from “above 3.4 when charging” to “Whenever above 3.4” and within 5 days, the voltages grew closer. Cells spent so little time above 3.4 while charging that there was very little time spent in equalization.

That worked for me.

Amazon has 16s active equalizers for between $55 and $75.
Actually i bought 2 of the heltecs but have not installed them yet as i had to wait for the winter to end so that I could disassemble the pack. It's encased in blue XPS foam with heating pads on the bottom aluminum plate as well as both of the side plates that compose the compression portion of the pack.... did not want to try that in the middle of winter.

so first try a rebalance and watch them for a month or two... if they start diverging again then I will take the two packs match strong against weak and use them as a 2p16s pack to see if they will hold a balance like that... last recourse is the heltecs.
 
The BMS has a passive balancer built-in, I believe
If it has any type of balancing, you can try that and just change the parameters on the BMS and let it be turned on more. That is what worked for me.

Passive balancing is bleeding off power in a cell with something like resistors instead of using it to charge a lower powered cell.
 
If your BMS has an active balance built in, the battery may not need to be set back in parallel for top balance.

28.8 charging is fine. Might just make it. And if all is good to 28.8 I would put direct in service and skip the top balance.

Sorry but what do you mean when you say, "might just make it"? Batteries are being delivered today and I want to give this my very best shot haha.
 
Sorry but what do you mean when you say, "might just make it"? Batteries are being delivered today and I want to give this my very best shot haha.
Just saying that the balance might be close enough to call it good right out of the box.

The BMS will protect either way. If the cells are well balanced the 28.80 could be reached just fine without a cell overvolt protection.
Could aim lower if concerned. 28.00 would be fine also and bump up from there.
 
Back
Top