24V working voltage range for Lithium Ion cells for a Van

[eatsleepvan]

Hi, we're designing an electrical system for a van that will have a 24V battery. We're using Boston-Power Swing 5300 cells, with a custom built BMS.

Each cell is nominally 3.65V, and we'd plan to charge each cell min 3.3V to 4.05V max. We can run the cells in 7 or 8 series. This would give us a working voltage range of:
26.4V to 32.4V with 8S
23.1V to 28.35V with 7S

Which voltage range would be best for most 24V electronics? e.g. LED lights, water pump, fuel pump (for diesel heater)? I'm assuming higher voltage would be slightly more efficient? Is 32.4V too high? Unfortunately the electronics themselves don't specify. I'm also going to have a DC-DC 24-12V converter, and that can handle 18-36V input.

[Sunking]

24 volt battery equipment is designed to work from 20 to 30 volts

[karrak]

32.4V is probably stretching things a bit too far. I would go for 7 cells.

Unless the pumps are electronically controlled they will use more power at higher voltages. If the LED lights just use resistors to limit current they will run at a higher power level and have a reduced lifespan. If they are powered by a switch mode power supply voltage won't make much difference.

Can you give some details on your BMS

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[eatsleepvan]

Thanks for the feedback. We're going to be using a BMS and display from http://zeva.com.au

[karrak]

A big thumbs up for the Zeva BMS, I assume you will be getting the BMS12. On paper it seems one of the best that I have seen. Please give some feedback on your system. Those batteries look interesting as well.

Thanks

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[Mike90250]

If you are using a BMS, be aware of it's limitations.
It has to be able to disconnect your battery under extreme high and low voltage. What happens to your charge controller if it's suddenly disconnected from the battery ?

if it uses bleeder boards, will it mange the amps your charge controller will be supplying, Say you are doing absorb at 8 amps, and your bleeder kicks in at only 6 amps. The other 2A are going into a full battery. Conversely, when a bleeder board fails (not a common thing) it shorts out, and drains one cell, destroying it.

Will the BMS interact with your inverter & charger to control them or do you need external contactors to do the job ?

[Sunking]

7S is your only option, but you have some interface issues to work out. 7S, or even 8S is not a great voltage range to interfaced with equipment made for conventional 12 volts. At 7S you are good on the high range being up to 30 volts. It is the lower range that could get you in trouble real fast. 12 volt inverters have built in Low Voltage Disconnects. They are set to trip at 21 volts. See the problem?

You are out of gas and running on empty at 26 volts, and the Inverter thinks 26 volts is a great voltage and keeps right on going until it quits working. You have a 5 volt incapability issue to deal with.

If you used LFP batteries are drop in replacement for the Lead Acid battery world to interface with. 8S Charge at 27.2 to 28.4 volts, and discharge to 21 volts. A 24 Inverter trips off at 21 volts. Perfect fit.

[eatsleepvan]

It has to be able to disconnect your battery under extreme high and low voltage. What happens to your charge controller if it's suddenly disconnected from the battery ?

if it uses bleeder boards, will it mange the amps your charge controller will be supplying, Say you are doing absorb at 8 amps, and your bleeder kicks in at only 6 amps. The other 2A are going into a full battery. Conversely, when a bleeder board fails (not a common thing) it shorts out, and drains one cell, destroying it.

Will the BMS interact with your inverter & charger to control them or do you need external contactors to do the job ?

Good points, so we're expecting to be able to make the BMS control the Victron inverter/charger and MPPT controller, to stop charging when limits are reached. We were also considering a Victron Battery Protect if needed which might be a good thing to have if the BMS fails for any reason.

[eatsleepvan]

7S is your only option, but you have some interface issues to work out. 7S, or even 8S is not a great voltage range to interfaced with equipment made for conventional 12 volts. At 7S you are good on the high range being up to 30 volts. It is the lower range that could get you in trouble real fast. 12 volt inverters have built in Low Voltage Disconnects. They are set to trip at 21 volts. See the problem?

Sorry not sure I understand, we'll be running 24V equipment off the battery, not 12. There is a DC-DC converter for 12V gear.

Our 24V-230V inverter is rated for 19-33V.
The 24-12V DC converter is rated for 16-35V
I am going to check the rest of the equipment we're planning on running, to see if the 8S voltage range will work.

The advantage for us using 8S6P, is we have 48 battery cells already (from another project). If we had 49 we could do 7S7P. But we're short 1, and you can't just add 1 to a series, so we'd have to do 7S6P thus loosing a good chunk of the battery.

[Sunking]

Sorry not sure I understand, we'll be running 24V equipment off the battery, not 12. There is a DC-DC converter for 12V gear.

Our 24V-230V inverter is rated for 19-33V.
The 24-12V DC converter is rated for 16-35V
I am going to check the rest of the equipment we're planning on running, to see if the 8S voltage range will work.

The advantage for us using 8S6P, is we have 48 battery cells already (from another project). If we had 49 we could do 7S7P. But we're short 1, and you can't just add 1 to a series, so we'd have to do 7S6P thus loosing a good chunk of the battery.

My bad, transposed numbers, I meant 24 volts. It does not really matter if 12 or 24 volts, your cell voltages are no compatible. If you have a battery that is completely drained by say 24 or 25 volts is not going to work well if the LVD of the Inverter is 16 to 19 volts. The Inverter is capable of destroying the batteries. If you go 8S with those batteries now you have both charge and discharge out of range. Most 24 volt Inverters will not like any voltage over 29 to 30 volts. All the Inverters are made to work with lead acid battery voltage ranges.

There is only 1 Lithium Ion Battery type compatible being LiFeP04 aka LFP. and uses 4S, 8S, and 16S. Not saying you cannot make it work, but you will have to figure out a way around it. Or use compatible batteries/ Take your pick. Expensive or more expensive is your choice.

[NEOH]

Why not use the higher voltage 8S Battery Bank?
Just add 26.4 - 30 Volt Input to 24 Volt Output Buck Inverters, as needed?

* Large amps
* Very efficient
* Low price
* Regulated 24 volts

[karrak]

Good points, so we're expecting to be able to make the BMS control the Victron inverter/charger and MPPT controller, to stop charging when limits are reached. We were also considering a Victron Battery Protect if needed which might be a good thing to have if the BMS fails for any reason.

Have you worked out a good way to turn off the inverter and MPPT controller remotely via the BMS? I had a quick look at the Vicron manual and it looks like it might be possible to turn the inverter off via the charger only/inverter on-off switch. I couldn't see any way to turn off the MPPT controller. As you have an electronic engineer helping, you could look at turning the MPPT controller off by modifying the temperature sensor so that BMS stops the charger by generating a battery over temperature fault.

Have you purchased a BMS from Zeva yet?

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[karrak]

My bad, transposed numbers, I meant 24 volts. It does not really matter if 12 or 24 volts, your cell voltages are no compatible. If you have a battery that is completely drained by say 24 or 25 volts is not going to work well if the LVD of the Inverter is 16 to 19 volts. The Inverter is capable of destroying the batteries. If you go 8S with those batteries now you have both charge and discharge out of range. Most 24 volt Inverters will not like any voltage over 29 to 30 volts. All the Inverters are made to work with lead acid battery voltage ranges.

There is only 1 Lithium Ion Battery type compatible being LiFeP04 aka LFP. and uses 4S, 8S, and 16S. Not saying you cannot make it work, but you will have to figure out a way around it. Or use compatible batteries/ Take your pick. Expensive or more expensive is your choice.

Yet another well informed and researched post from Sunking.

The OP has already stated he is using a Victron Inverter/MPPT controller. If you look at the manual for this you will see that the LVD is programmable and the maximum battery voltage is 33 volts. Even my 24V inverter which was probably designed around 15 years ago has a set of switches to set different LVDs up to 24V and will operate at voltages up to 34V.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[karrak]

The advantage for us using 8S6P, is we have 48 battery cells already (from another project). If we had 49 we could do 7S7P. But we're short 1, and you can't just add 1 to a series, so we'd have to do 7S6P thus loosing a good chunk of the battery.

If you are wiring the battery with seven cells in parallel and then wiring the seven cell blocks in series although not ideal I would think that adding one cell if the other 48 cells haven't lost too much capacity would not be an issue. Any capacity difference between the used cells and the new one will be reduced to by a factor of 7 because it is only one of seven cells. As an example a 10% difference in capacity between the new and used cells will result in only a ~1.4% difference between this block of seven cells and the others blocks in the battery.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[karrak]

Can I just clarify something, by 8S6P do you mean six of the Ensemble 115 8p1s modules from Boston Power so your battery would really be 48p8s which works out at ~7.44kWh of storage?