My latest battery build

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jonescg
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My latest battery build

Post by jonescg » Sat, 03 Nov 2018, 11:17

This is a nominal 355 volt, 10 Ah battery built using 5 Ah pouch cells (96s2p). It's for the UWA formula student team's race car. The intention was always to use this battery as the dyno-testing and bench-tuning battery given it's capacity, but if push comes to shove, then can drop it into the car and get rolling.
Pack finished top.jpg
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Pack finished front.jpg
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It's using the 30C LiPo (lithium cobalt oxide) pouch cells I normally use in the race bike, so they are capable of some serious short-lived power. This 25 kg battery is able to deliver 100 kW, albeit briefly.

Being a high voltage battery it has numerous designs built in to make it safer. All four 24s units are physically separated by 4.5 mm thick polycarbonate dividers. Two EV-Power BMS modules manage 12 cells each, and are connected by an IsoSPI comms bus. The centre of the battery is where the main 200 A fuse is located, and it is easily removed for pack service. In the vestibule is all the hardware - main contactor, precharge relay, 50 W, 200 Ohm pre-charge resistor and a negative contactor. There's also a 10 A fuse for the charge circuit. The negative contactor also serves as an isolation point for the charge circuit.

It's not waterproof, but it could be made so. It's also not actively cooled, however you could feasibly put some fans on the top cover and draw any hot air out. Being such low impedance cells, the pack isn't expected to generate a lot of heat under the duty cycles they are likely to put it through.
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Re: My latest battery build

Post by soyachips » Tue, 15 Oct 2019, 21:34

I know this thread is old but very nice looking pack!!!

I like how the Anderson connectors are mounted against the box. I'm rethinking how to do mine now.

Also what's a negative contactor and what did you mean about it serving as an isolation point for the charge circuit?

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Re: My latest battery build

Post by brendon_m » Tue, 15 Oct 2019, 22:38

A negative contactor is just a contactor on the negative (0v) line out of the pack. The idea being it allows you to isolate both positive and negative outputs and it's safe to touch either terminal and there are no hazardous voltages without the system being powered up.
In terms of isolating the charger, the positive charger connection goes straight to the pack and negative goes through the contactor so you can isolate the charger and engage it but not connect power to the whole vehicle

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Re: My latest battery build

Post by soyachips » Wed, 16 Oct 2019, 07:08

brendon_m wrote:
Tue, 15 Oct 2019, 22:38
A negative contactor is just a contactor on the negative (0v) line out of the pack. The idea being it allows you to isolate both positive and negative outputs and it's safe to touch either terminal and there are no hazardous voltages without the system being powered up.
In terms of isolating the charger, the positive charger connection goes straight to the pack and negative goes through the contactor so you can isolate the charger and engage it but not connect power to the whole vehicle
Awesome thanks for the explanation ... more rethinking of my pack required! :lol:

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Re: My latest battery build

Post by jonescg » Wed, 16 Oct 2019, 09:57

Also in this instance the team required two points of isolation - a contactor on both terminals - and some means of interrupting the circuit should the BMS warn of a cell out of spec. The negative contactor served this purpose as well. It's an awfully large switch for the ~10 amps it's supposed to be interrupting, but made good use of it. Most of the time chargers come with an Enable line which can be opened by the BMS should a cell exceed 4.2 V. That's what I've done on this latest pack.
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Re: My latest battery build

Post by soyachips » Sun, 20 Oct 2019, 18:00

So I guess the charger realises the pack is no longer connected and stops charging? The charger I have doesn’t come with an enable line so I’m planning on using a relay that has 12V DC on one side and 240V AC on the other to turn off the charger if any cell goes too high. Can you see any issues with this?

I like the way you build your packs using polycarbonate and screws but was wondering if they are strong enough on their own or if they go into a stronger box?

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Re: My latest battery build

Post by coulomb » Mon, 21 Oct 2019, 07:49

soyachips wrote:
Sun, 20 Oct 2019, 18:00
The charger I have doesn’t come with an enable line so I’m planning on using a relay that has 12V DC on one side and 240V AC on the other to turn off the charger if any cell goes too high. Can you see any issues with this?
There are those, including myself, who don't like regularly interrupting the input of a power factor corrected power converter like a charger. The AC input, after the rectifier which usually has two diodes conducting hard, goes straight into an inductor, which stores considerable energy in its magnetic field. That energy, which under normal circumstances ends up in the battery being charged, has nowhere to go when the AC input is interrupted. So there are some components, or the contact opening the AC input, that has to absorb the energy of the inductor. These can eventually wear out and cause problems. Many chargers seem to use under-rated components to absorb this energy.

For this reason, I don't like the idea of a timer to turn off the charge after a certain time every charge. In your case, however, it sounds like it will only happen when a cell goes too high in voltage, which might be a few weeks in a row, but not every charge, so it might be acceptable.
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Re: My latest battery build

Post by soyachips » Wed, 30 Oct 2019, 03:38

Thanks @coulomb, I’m using the 72V Satiator charger from Grin Technologies so will check with them if this will be an issue or not. Also need to consider what happens when a cell hits HVC and then drops down as I don’t think the BMS has a latching relay. More food for thought :D

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