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

dianea

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I wish these batteries were designed for high voltage configurations. But I know when one charge controller attempts to open up for overload conditions for any reason, it is likely to see a rude event of full voltage across that poor semiconductor that wasn't rated for it. And any communications port is going to see a wild voltage differential at all times. The case is going to be charged with a potential beyond design specifications.

A 600 volt battery setup for an EV would be nice. But one 48 volt charge controller opening up for a moment is going to explode. And the energy released will vaporize metal all over, further discharging massive energy stores in a huge arc flash. As someone that maintained substations in large manufacturing plants, this is the ultimate event. This is when big property damage happens. And if someone is in the area, instant goodbye!
 

zstine

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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.
It's a catamaran and will be a live-a-board and not that expensive comparing the cost of just an 80kWh battery pack! I have looked at 48V motors and they seem to top out at about 40hp/30kW, which would be marginally okay. But I'm under the impression that 48V drive would be significantly lower in effeciency, which means I have to accept a shorter range and if I can design around that, I'd like to. I haven't quantified the difference in eff between 96V and 48V though. maybe it's not that significant? yes, AC as I want to be able to regenerate in the future.
 

zstine

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So just to clarify... Theoretically, is there any reason I can't use a 30S 96V BMS and use two 48V chargers one the 1-15S cells and one on the 16-30S cells? I guess I need two BMS circuits, one for each charger connection... not sure how to connect that.?
I understand that balancing cells may be an issue, but wouldn't they all 'equalize' when charged to 100%? Yes, If I'm pulling an inverter load off just one of the chargers/inverters (15cells) than those would discharge more deeply. But If I'm recharging to 100% daily, is that really a big deal considering my inverter load imbalance is expected to be small <5% compared to total capacity. In other words, I would never expect one 15cell group to have more than 4kW of discharge imbalance in an 80kW pack... insignificant???
 
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Ampster

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My experience with displacement hulls is there was a tradeoff between RPMs and speed and a sweet spot for efficiency. Some of that also has to do with prop pitch. i put an AC50 in a VW conversion and know those to be fairly efficient. I don't know if the efficiency loss you speak of is related to the Curtis's controller or the motor windings. I assume the AC20 is from HPEVs and perhaps they or your vendor can give you some specifics. There is a 48 volt version of the Ciurtiss controller and maybe HPEVS make a motor with a winding that has a narrow more efficient RPM range suitable for propulsion on water.
I agree that 48 volt inverters and charge controllers are going to be easier to find.
 
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Ampster

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Theoretically, is there any reason I can't use a 30S 96V BMS and use two 48V chargers one the 1-15S cells and one on the 16-30S cells? I guess I need two BMS circuits, one for each charger connection... not sure how to connect that.
That is the dilemma. I do not know of any situations where a pack has been used like that successfully. It is a good way too trash an expensive pack by pulling down half of it with no way to balance the 48 volt segments. The BMS could take months. The faillure of my pack because half of it has sucked down the other half is not something I would want to encounter in the middle of the Ocean.
If I wanted to brainstorm this, I would look at various scenerios and costs for 96 volt systems and two 48 volt systems with one or two motors. It is a catamaran and weigh balance is always better on a boat if you can keep the weight low in each hull. There has to be some value in redundancy.
 
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Ampster

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@zstine
Here is the 48 Volt chart for the AC-20. I do not know what RPMs or prop pitch it would take to move your catamaran at a reasonable cruising speed but this might help if you know those inputs.
 

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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.
Hi, at first read i was thinking 'yeah, the BMS has to have 96V across it". But now I don't understand why. If you have two 48V batteries in series, you still only have 48V potential across each battery, right? zero to 48V across the first 15S and 48V(-) to 96V(+) in the second. The BMS would have to carry the total discharge Current when in series but only 48V. Clearly I’m missing something. Can you explain when/how the BMS FETs would ever see 96V?
That said FET overload is a non-issue if you don't use them... see below. Maybe an expensive option with less features though.1655498647119.png
1655498647119.png
 

Hedges

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One 48V battery with resistive load.
BMS opens, resistor pulls voltage down, FETs get 48V across them.

Two 48V batteries in series, with a resistive load.
One BMS opens. Resistive load pulls voltage past the rail of that battery to rail of other battery. FETs get 96V across them.

If you had a 2-cell flashlight and pulled the two batteries apart (opened a BMS switch between them), 3V would appear across that.
 

zstine

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One 48V battery with resistive load.
BMS opens, resistor pulls voltage down, FETs get 48V across them.

Two 48V batteries in series, with a resistive load.
One BMS opens. Resistive load pulls voltage past the rail of that battery to rail of other battery. FETs get 96V across them.

If you had a 2-cell flashlight and pulled the two batteries apart (opened a BMS switch between them), 3V would appear across that.
When the BMS opens, or you pull the batteries apart in a flashlight, doesn't that make an open circuit? The current and voltage drops to Zero, like turning off a switch, since there's no longer a complete closed loop? Each battery would have their own respective votage across the posts (48v or 1.5v) but there's nothing flowing in the circuit.
EDIT.. I think I understand, there's 3V or 96V aross the gap... oh duh.. i'm being dumb
 
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