Two 48V banks in series for 96V with Off-The-Shelf Equipment

zstine

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I am planning a solar/electric powered boat. The plan was a pair of 6kW arrays, each running through a EG4 6500W inverter/charger to a pair of 40kWh, 48V banks. The problem is the 55hp, AC-20 motor is to be run at 96V. So, I THOUGHT I could run the two 48V banks in series BUT EG4 support says nope. Won't work with their BMS and you'd likely get charger faults.

Can I use a pair off the shelf 48V chargers (like LV6548) to charge a pair of 48V banks connected in Series to power a 96V motor? Or will all the off-the-shelf inverter/chargers and/or BMS end up with faults/errors/shutdowns? I don't want to spend $30k and have it not work.

The split phase 240VAC you can get from having the 2 inverter/chargers communicating is nice to have, but I can deal with only having 120VAC if it would be required to have the units isolated.

Thanks
zach
 
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rmaddy

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The problem is that you can't put 2 48V LiFePO₄ batteries in series unless the batteries have a BMS that supports being put in series. I don't think there are any that support being in series at 96V. You likely need to find a different chemistry (lead-based perhaps) that allows the creation of 96V battery banks.
 

zstine

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The problem is that you can't put 2 48V LiFePO₄ batteries in series unless the batteries have a BMS that supports being put in series. I don't think there are any that support being in series at 96V. You likely need to find a different chemistry (lead-based perhaps) that allows the creation of 96V battery banks.
Can we dumb it down? The electrons in the wires don't care what the batt chemistry is. My Victron charger has LiFePO charge profile and no 'smart BMS' connection. While I understand it is stupid and dangerous, theoretically you can charge the batteries with no BMS...
Do the Chargers care if they are charging only half of a series bank? Essentially you are connecting the chargers in series... will that work?
 

JoeHam

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It doesn’t sound like you’re headed in the right direction.

Take a look at this thread :

 

rmaddy

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Here's what is likely to happen when you charge LiFePO₄ batteries without a BMS:

 

zstine

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OK I answered my reply to rmaddy above... Yes per batterytender.com see below... So How do I safely/cheaply do this with a pair of 'standard' 48V batteries and a pair of 'standard' 48V chargers?
1655415293716.png
 

rmaddy

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But you can't put 2 48V LiFePO₄ batteries in series. Charging them is a moot point.
 

zstine

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Here's what is likely to happen when you charge LiFePO₄ batteries without a BMS:

I wouldn't charge without BMS.
I guess my issue is understanding why the BMS cares what's outside the battery. It knows is what current and voltage is at the +/- posts, and battery internals... So why does it matter to the BMS if the battery is series or parallel? Doesn't it just see a different condition at the +/- posts in each config and do it's current & charging calc's, etc.?
But you can't put 2 48V LiFePO₄ batteries in series. Charging them is a moot point.
What!???? I've seen plenty of 96V LiFePO packs. They all use 3.2V cells in various parallel and series configurations to get 96V. So why can't you run two 48V LiFePO in series?
 

rmaddy

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I wouldn't charge without BMS.
In post #3 you stated:
While I understand it is stupid and dangerous, theoretically you can charge the batteries with no BMS..
I was simply pointing out the real life danger of charging without a BMS since you brought up the idea.

So why does it matter to the BMS if the battery is series
Because many BMS can't support the higher voltage going through it. There are many LiFePO₄ batteries at 12V, 24V, or 48V that do not support being put in series at all.

So why can't you run two 48V LiFePO in series?
In post #1 you stated:
I THOUGHT I could run the two 48V banks in series BUT EG4 support says nope.
In post #2 I stated:
The problem is that you can't put 2 48V LiFePO₄ batteries in series unless the batteries have a BMS that supports being put in series.

If you can find a 32S BMS that supports building a 96V LiFePO₄ battery from 32 3.2V cells then you can have a 96V LiFePO₄ battery. Or if you can find two 48V LiFePO₄ batteries that have a BMS that supports being put in series then you can have a 96V battery.
 

LithiumSolar

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What!???? I've seen plenty of 96V LiFePO packs. They all use 3.2V cells in various parallel and series configurations to get 96V. So why can't you run two 48V LiFePO in series?

The FETs on the BMS that control charging/discharging on and off need to be rated for the maximum string voltage. If you series two 48V batteries, that's a maximum of 116.8V at full charge. If the FETs are out of spec, you risk them not switching, getting stuck "on", and/or blowing out. I'm not aware of any 48V batteries on the market that state they can be wired in series.
 

Ampster

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The simple solution is to build a 32 cell battery with a BMS designed for 32 cells. Also get the correct charger for 32 cells. Visit an EV forum for ideas. It is done all the time with EV conversions. I presume you are using a Curtis controller with the AC20? You can probably do it less expensively than two EG4s and have more flexibility in configuring them in a boat. Forget about a FET based BMS. All the EV conversion guys only use contactor based BMSs because of safety and Amps. You also should spend the extra money for Automotive grade cells because they will take high discharge rates without voltage drift.

The vendor where you are purchasing the AC20 probably has BMSs for 32 cells.
 
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zstine

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The FETs on the BMS that control charging/discharging on and off need to be rated for the maximum string voltage. If you series two 48V batteries, that's a maximum of 116.8V at full charge. If the FETs are out of spec, you risk them not switching, getting stuck "on", and/or blowing out. I'm not aware of any 48V batteries on the market that state they can be wired in series.
Okay, The problem we are having in communication is semantics and my fault probably. I don't consider the BMS as part of the "battery", since for ex. lead acid batteries don't include BMS. I generally think of them as 2 parts, where the BMS is connected to the battery. The battery being a group of connected cells. So, if you think of it this way, then there is no reason why you can't connect two 48V batteries in series... But in order to be safe w/ LiFePO, you need to find a BMS which can handle the voltage, which I think can simpley be a 96V LifePO BMS. like maybe this https://lws-pcm.en.made-in-china.co...MS-96V-30s-200A-250A-LiFePO4-Battery-BMS.html
 

zstine

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The simple solution is to build a 32 cell battery with a BMS designed for 32 cells. Also get the correct charger for 32 cells. Visit an EV forum for ideas. It is done all the time with EV conversions. I presume you are using a Curtis controller with the AC20? You can probably do it less expensively than two EG4s and have more flexibility in configuring them in a boat. Forget about a FET based BMS. All the EV conversion guys only use contactor based BMSs because of safety and Amps. You also should spend the extra money for Automotive grade cells because they will take high discharge rates without voltage drift.

The vendor where you are purchasing the AC20 probably has BMSs for 32 cells.
Yes, doing everything in 96V is possible but maybe not so simple/inexpensive. The EV cars use AC chargers connected to the grid. I'm charging with DC solar. I am not aware of inexpensive 96V MPPT inverter/chargers like you can buy for 48V. And I'm looking at 12kW to 16kW of solar, so I will need multiple solar charge controllers, exacerbating the increase in cost, if 96V MPPT chargers are even available. I will also need to supply 120VAC, 12VDC and 24VDC to the boat, which looks easier starting with a 48VDC battery bank vice a 96V b/c of the commonality of 48V systems. Cost and serviceability often comes down to keeping within the popular/common configurations available.
EDIT: a quick google does show MPPT 96V chargers are available, without inverting though... I can add that separate if it's better than getting a 96V BMS for a pair of 48V batteries
 
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Ampster

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I'm looking at 12kW to 16kW of solar
That is a lot of square footage. At 20 Watts per square foot that is 600 to 800 square feet. Is this a houseboat?
If you can afford a boat that big you should be able to find a charge controller that can handle 96 volts. There is a KISS principle in electronics which probably applies on the water when away from shore. Cost and serviceability also includes the challenge of keeping two 48 volt packs balanced.
Have you priced 48 volt motors? That might be a compromise that would make all the other components more reasonable. Does it have to be AC? The motor voltage is the issue which is making this complicated.
 
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foggysail

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But you can't put 2 48V LiFePO₄ batteries in series. Charging them is a moot point.


I know poop about LiFePO4 batteries! And 48V LiFePO4 batteries are ''smart enough'' to know they are in series???? Really? There must be more to this.
 

rmaddy

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I know poop about LiFePO4 batteries! And 48V LiFePO4 batteries are ''smart enough'' to know they are in series???? Really? There must be more to this.
Read posts #9 and #10 of this thread. There are also other threads on this forum discussing why many BMS don't support being put in series.
 

foggysail

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OK guys--- that makes sense. I can see how correctly equalizing each battery could be an issue.
 

Hedges

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You may get away with connecting two sources (batteries) in series even though they aren't designed for the voltage if you put reverse-polarity clamping diodes across them.

I have done that, at low amperage, for switch-mode supplies providing +/-15V on a PCB. The problem had been power-up sequencing where one delivered 15V first and through the load the other one was pulled across the rail. A Schottky clamp diode across each output prevented that. Same for HP bench supplies; during power-up, output pulled across the rail (to wrong side of ground) caused other supply to give error "over" voltage.)

For high power you would use silicon junction diode, and it is possible that isn't fast enough (could allow a large voltage excursion before conducting.) My low current circuit I was able to use fast Schottky. Other circuits might do the job but would be more complex than just a diode.
 
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