Charge a 7s LiPo as though it was a 24v lead battery?

[kalisiak]

Hello all,

I don't think this is particularly dangerous, but it's not typical, so I figured I would post it here.

I'm wondering if there's any reason why you couldn't pretend that a 7s LiPo is actually a 24V lead battery. Looks like from a bit of Googling that the absolute maximum voltage that a 24V lead charger will put out is 29.4V during bulk phase in constant current, and then that same voltage during absorption phase in constant voltage. Eventually the 7s group will stop accepting current during absorption, and the charger will think the pack is fully charged.

29.4V is exactly 4.2V*7, so if your pack is BMS protected, and every single lithium cell in the pack is BMS protected, it seems like it should be safe? The cells I'm using are rated as fully charged at 4.35V, so even 4.2V does allow a little bit of headroom, and the overall BMS would kick out if 29.4V is exceeded anyway.

Usually the downfall of a lead charger is the float, but even then, it's going to kick in at ~3.85V/cell, and will replenish what was consumed by the load that's attached to the pack. And if the pack has a constantly draining load, and is only charged when there's insolation, it shouldn't stay in an excited state for very long, I wouldn't think.

Thanks,
Chris

 

[yabert]

I have a 7S battery with NMC cells (4.2V to 3.0V range).
7S is perfect, exactly like 14S is perfect to replace 48V lead, but despite that I would stay away from lead charger.
Many others constant current/constant voltage device can be use to charge lithium.

 

[kalisiak]

Is there a reason why you would stay away from a lead charger? The details of the charge characteristics aren't the same, but there doesn't seem to be anything incompatible with a 7S pack as far as voltage is concerned, and the total pack will actually be 2x(BMS7s@BMS30p), and the charge current will consist of 600W worth of solar panels, so the current can't stress the pack either.

Basically, I have an ECO-WORTHY 3000W 24VDC to 110V Solar Charge Inverter, two 36V 300W panels, and ~5kW of lithium, and am trying to figure out if there's something that is actually incompatible at 7S.

Thanks,
Chris

 

[Checkthisout]

Is there a reason why you would stay away from a lead charger? The details of the charge characteristics aren't the same, but there doesn't seem to be anything incompatible with a 7S pack as far as voltage is concerned, and the total pack will actually be 2x(BMS7s@BMS30p), and the charge current will consist of 600W worth of solar panels, so the current can't stress the pack either.

Basically, I have an ECO-WORTHY 3000W 24VDC to 110V Solar Charge Inverter, two 36V 300W panels, and ~5kW of lithium, and am trying to figure out if there's something that is actually incompatible at 7S.

Thanks,
Chris

Doesn’t matter as your wimpy lead acid charger can't throw enough amps at your lithium to matter.

The absorption time with la isn't necessary so just turn the charger off once it's 14.4-14.6 for about a 1/2 hour

 

[yabert]

Is there a reason why you would stay away from a lead charger?

Voltage range. If it isn't adjustable for HV and LV limit, I would stay away. You should adjust the charger to match the battery limit and the BMS should only be there to stop any unrespect limit by the charger. It should not act as a charge limiter.

Basically, I have an ECO-WORTHY 3000W 24VDC to 110V Solar Charge Inverter, two 36V 300W panels, and ~5kW of lithium, and am trying to figure out if there's something that is actually incompatible at 7S.

Almost the same set-up than me. I simply use the User-defined option of my PowMr inverter charger to adjust the voltage limits (HV and LV).
It seem that your Eco-Worty and my PowMr are clone of Voltronic or MPP inverter, so your is probably also adjustable.

 

[kalisiak]

Doesn’t matter as your wimpy lead acid charger can't throw enough amps at your lithium to matter.

The absorption time with la isn't necessary so just turn the charger off once it's 14.4-14.6 for about a 1/2 hour

The Eco-Worthy unit is an all-in-one, actually, with only the ability to switch between "AC primary" and "Battery primary".
- When it's on "AC primary", it's basically a 3000W UPS, and will use AC to charge the batteries, or solar panels in PWM, but other than that the AC is passthrough.
- When it's on "Battery primary", it's a 24V inverter and the 36V solar panels are engaged in PWM to provide power to the batteries/inverter. It falls back to AC power if the batteries become depleted, and then the batteries just sit there in the depleted state until the sun rises.

You have to turn the unit off in order to switch modes, as far as I have been able to tell, so babysitting this thing to turn it off and switch modes when it's full, and turn it back on again, isn't in the cards.

I was asking because none of the voltages that a 24V charger emits seems like it should be a concern. The 24V lead cell charging voltages are less than or equal to 4.2V/cell, and the float voltage is like 3.85V per cell, so I expect nothing would happen with a 7s LiPo if it's already charged more than that.

With this said, I haven't decided how I'm going to deploy this thing yet. I bought it through FB marketplace last weekend, and I've been experimenting with a pair of 26Ah SLAs and a pair of 300W panels to see how this thing behaves. It sure is ham-fisted, and it's not configurable. The small APC BackUPS that I have plugged into it is quite cranky about being there, but not so upset that it kicks over to its battery. I don't know if it's because the "pure sine wave" is a false advertisement, or if it's because ground isn't connected through the unit, or something else.

Thanks,
Chris

 

[kalisiak]

Voltage range. If it isn't adjustable for HV and LV limit, I would stay away. You should adjust the charger to match the battery limit and the BMS should only be there to stop any unrespect limit by the charger. It should not act as a charge limiter.

Almost the same set-up than me. I simply use the User-defined option of my PowMr inverter charger to adjust the voltage limits (HV and LV).
It seem that your Eco-Worty and my PowMr are clone of Voltronic or MPP inverter, so your is probably also adjustable.

Understood, I know the BMS is not a charge controller, but the voltages that a 24V lead charger emits aren't anything that would cause a 7s 30A BMS to trigger. The upper end of the charging voltage from a 24V lead charger are less than or equal to the 7s max, and even the float charge at 3.85V/cell shouldn't do anything to a 7s pack that's already charged.

The unit I have is the first generation Eco-Worthy 300W all-in-one, with the carrying handle, actually, so nothing about it is configurable. I bought it from someone for $160 last weekend. It seems to work, but over the last week that I've been playing with it, I'm not really impressed. I might just use it as a 3000W inverter.

The ultimate goal, which I still haven't figured out how to do yet, is tie the 36V panels on the roof of the 12/3 romex detached garage, to a 5kW 7s pack in the basement of the house, with an inverter to provide power to critical loads during a power failure. And to make use of the garage panels when the mains is up...

Thanks,
Chris

 

[yabert]

The upper end of the charging voltage from a 24V lead charger are less than or equal to the 7s max, and even the float charge at 3.85V/cell shouldn't do anything to a 7s pack that's already charged.

Ok, nothing wrong, it should work.

Simply double check everything, manually monitor the end of the charge and be prepare to take action.

I mean, Lipo cells are not recognize to give a second chance during over voltage event... read here your house in fire.
Do you know the exact chemistry of your cells? Or the model?

I have 7S NMC cells coming from Ford Energy vehicle in my house and I know they can be dangerous and it's why I use 3 protection level.
1-Inverter HV/LV adjustment
2-BMS HV/LV per cell
3-A HV/LV device who stop BMS if this one don't take action

In your case you seem to rely on the goodwill of a cheap non adjustable inverter and a BMS to save your house, so take care and stay safe.

 

[kalisiak]

Ok, nothing wrong, it should work.

Simply double check everything, manually monitor the end of the charge and be prepare to take action.

I mean, Lipo cells are not recognize to give a second chance during over voltage event... read here your house in fire.
Do you know the exact chemistry of your cells? Or the model?

I have 7S NMC cells coming from Ford Energy vehicle in my house and I know they can be dangerous and it's why I use 3 protection level.
1-Inverter HV/LV adjustment
2-BMS HV/LV per cell
3-A HV/LV device who stop BMS if this one don't take action

In your case you seem to rely on the goodwill of a cheap non adjustable inverter and a BMS to save your house, so take care and stay safe.

Each individual cell is unused, and has its own discrete BMS on it. The 420 cells (so that means four hundred twenty individual BMS circuits) are in two 7s30p groups, each group with a 7S 30A BMS. They will be stored in a large heavy-steel ammunition box with an L14-30 penetrating the side, passing through a a 30A three-conductor contactor to the L14-30R. The contactor will be driven by the lack of 120V AC power, a low-voltage cutoff circuit set to 21V, and a temperature controller in it set to 35 degrees, such that if either it reaches low voltage or if it starts to excessively heat up, the contactor will disengage. So I'm not worried about the inverter doing anything funny, which is likely only going to be run at 30-50% duty cycle.

After playing with it this week, I'm not super happy with how this Eco-Worthy behaves, so it's not likely that the Eco-Worthy will have the responsibility of charging it up on a regular basis. It can certainly charge the pack with supervision, if need be. The basic idea is that I will only be using this as an emergency UPS, so the batteries will be powering only a simple voltage monitor 24/7, with a starting point of 90% SOC. The Eco-Worthy will generally be receiving no power from either AC or battery unless there is a loss of AC. I might need to top them back off to 90% once every month or two, but I'll figure that part out when I get there.

Chris

 

[Ampster]

I just need to throw in a caveat if you are actually talking about Lipo and not one of the other 3.7 V nominal chemistries. Lipo is the most volatile of all the chemistries but it does have its advantages for the hobbyist remote controlled devices. Those batteries are often small so the consequences of a fire are limited and they are often charged and transported in fireproof bags.

 

[kalisiak]

I just need to throw in a caveat if you are actually talking about Lipo and not one of the other 3.7 V nominal chemistries. Lipo is the most volatile of all the chemistries but it does have its advantages for the hobbyist remote controlled devices. Those batteries are often small so the consequences of a fire are limited and they are often charged and transported in fireproof bags.

Yes, actually, I just check the spec sheet again, and they are, in fact, LiPo.

I'm planning on driving them at a maximum of 1/4C (they're rated at 4500mAh and 1C, but the overall BMS won't let me pull more than a maximum of 1A from each) only a couple times a year for critical loads when there's a power failure, and I don't have any plans of crashing them into a wall.

The ~7kWh worth of cells will be hovering around 80% SOC while sitting idle waiting for said power failure, and I won't take them below 20%, so that gives me about 4kWh to work with. When they drop below like 75% SOC, I'll likely use something like a 1A 7s charger to top them back off again. They're rated for 500 cycles, but if I capacity test them down to 20% and back up to 80% once a year, they'll last quite a while.

Again, these are brand new cells, manufactured about two years ago, never used, each cell with their own BMS, broken into two groups of 7s30p each with decent-grade 7s BMS, sitting in a heavy-gauge locking steel ammunition box. Temperature and voltage monitoring will disengage the 2x8GA pairs if something goes wrong while using them, and the 3-pole 30A contactor will only feed power to the inverter when I want it to (e.g. when AC power is lost, and the NO relay closes and feeds power to the contactor).

I realize that anything short of LiFePo has risks, but I don't see that there is a lot of danger involved here.

Chris