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Old 10-24-2019, 06:24 AM   #21
DaveTM
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Default Re: first step in DS conversion to lithium seems easy

Just to add (and I have an aftermarket controller and motor) that my cart will draw 300+ amps when I'm starting up a steep hill. Granted, it may be for a few seconds....depending upon the hill and how much weight is in the cart.
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Old 10-24-2019, 07:35 AM   #22
dlwilson
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Default Re: first step in DS conversion to lithium seems easy

Sergio's suggestion above takes the load off the BMS. In his scenario the BMS just controls a relay that's inline with the keyswitch, and the current does not flow through the BMS. When the BMS detects under-voltage it de-energizes the relay and shuts off the cart. That means I could have bought a lower-capacity BMS (as long as it handles enough amps for charging).

Hopefully that description is correct.

I did wonder when I bought the BMS if 150 amps would be enough. I have a stock controller now, but would eventually like to get an Alltrax 400 or 500. South Florida is flat, but more speed and acceleration is always good.
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Old 10-24-2019, 08:10 AM   #23
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Default Re: first step in DS conversion to lithium seems easy

The 3.0v and 4.2v are the threshold limits for those cells, so ideally a range of min 3.3v to max 4.1v would be better for the battery life.

Your charging voltage may be an issue, according to the description if the cells are overcharged (4.2v ±0.05v) the BMS will not release the lock until the voltage has dropped to less than 4.05v (±0.05v) per cell.

That means 48.0v to 49.2v for the pack to release, so charging to 50.4v may trigger the over protection on the low side of the overcharge margin of 49.8v and since the BMS shunts can only drain 0.06A, it could take a while before the BMS releases the lock.

A lot of those inexpensive lithium power supplies share the same board and there are a lot of posts on the internet identifying the trim-pots on the board that control the max voltage setting so perhaps you can lower the final voltage a little in case the charger triggers the over-voltage protection of the BMS.

Once You get Your Alltrax Controller You can use the Controller low and high voltage limits so the controller can automatically reduce the current demand if the battery approaches the low voltage cuttoff and the high limit if the cart is a Sepex with Regen brakes.
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Old 10-27-2019, 08:41 PM   #24
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Default Re: first step in DS conversion to lithium seems easy

This afternoon I got the chance to mount the Volt battery in the cart. I hooked up the voltage display, but nothing else (no BMS). It was at 48.2V when I started, and I took it around the block a few times. Acceleration was much improved, and top speed was 18 mph, which is what I got from the SLA batteries that came out.

But after about a mile the voltage was down to 47.5 and top speed sagged a bit. Is that because I started off with a low voltage? Should I have charged it all the way up to 50V? Or is it my used battery?

I did a lot of reading trying to learn how to choose and operate a lithium battery, but couldn't get a clear idea. So I just jumped and bought a battery, BMS, and charger, and figured that was the price of learning. Next step is adding the BMS and relay system that Sergio described above to ensure it doesn't over-discharge. Is there anything else I need to be doing to monitor the battery during charging or discharging?
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Old 10-28-2019, 07:42 AM   #25
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Default Re: first step in DS conversion to lithium seems easy

Quote:
Originally Posted by Sergio View Post
The 3.0v and 4.2v are the threshold limits for those cells, so ideally a range of min 3.3v to max 4.1v would be better for the battery life.
So after thinking about this for a while, if 3.3v is my min voltage per cell, I can discharge the battery pack to (12 * 3.3v) or 39.6v conservatively? Of course, the BMS is necessary to ensure that no individual cell drops below 3.3v.
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Old 10-28-2019, 08:26 AM   #26
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Default Re: first step in DS conversion to lithium seems easy

In theory yes, in practice the voltage changes much faster at the lower portion of the discharge curve for lithium batteries, the capacity difference between 3.3v and 3.0v per cell is about 2AH or 2 minutes at cruising speed.

A user programmable controller like the Alltrax would allow you to setup the low voltage cut-off to 39.6v so the Controller would automatically reduce the current if the voltage starts to get closer to that value (basically it creates a limp mode for low voltage).

If using the OEM controller, just go easy on the acceleration once the "not moving" pack voltage gets down to 42v.
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Old 10-28-2019, 11:01 AM   #27
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Default Re: first step in DS conversion to lithium seems easy

Sergio sounds correct on all this. I wanna do my Western as soon as Dave Dives up the Lake Worth Bridge, recharges without burning down the church next door or taking down the downtown grid
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Old 10-28-2019, 11:11 AM   #28
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Default Re: first step in DS conversion to lithium seems easy

Unless otherwise specified by the specific BMS product, all a typical BMS does is disconnect the entire pack when any individual monitored cell goes below or above the set value.
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Old 10-28-2019, 11:17 AM   #29
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Default Re: first step in DS conversion to lithium seems easy

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Originally Posted by mederwood View Post
I think the BMS only requires voltage at or slightly above the battery terminal voltage or whatever the BMS spec requires. I don't think you need a charger, just a A/C to ~48 VDC power supply. I think the BMS takes care of the regulation, balancing and protection. Dose this sound correct?
I'm not quite sure what you're asking...or stating. If you mean any old 48V charger can be used on lithium that isn't a correct statement. Perhaps a 48V LA charger "can" be used.....it may not be the best for long term charging of a lithium pack. It's my understanding that lithium specific chargers are "out there" so the correct power to the lithium battery(s) charge them to the set maximum, without any damage that could effect the useful life of the pack.
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Old 10-30-2019, 09:17 AM   #30
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Default Re: first step in DS conversion to lithium seems easy

I've been running the cart around town, trying to get a feel for how reliable it is and what the range is. Yesterday at lunch I put on 1.99 miles. Voltage dropped from 49.2V to 48.2V. so 0.5V per mile. I have no idea if this will be consistent throughout the voltage range. I drive at full throttle, no regeneration.

Next step is to build a more robust battery mounting system (that isn't held in with c-clamps), then wire in the 48V-12V reducer and 48V relay so I have headlights and brake lights.

What I really want to do is use the BMS wiring to check the voltage of each cell, which is something I just haven't had time to do. I'm really curious to know how close they are.
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