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Unread 11-21-2019, 12:57 PM   #41
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Yamaha
 
Join Date: May 2012
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Default Re: first step in DS conversion to lithium seems easy

Inbalance of 0.3V is a lot especially if the battery is up in the 3.7V or higher range. I would manually balance the cells up at around 4V. You can discharge the higher ones or charge the lower ones. The balancing circuits in most BMS's can't easily deal with that big an imbalance. For my RC batteries I use a car halogen headlamp bulb to discharge them manually. You have to monitor the voltage while doing so. My Leaf cells have never been very imbalanced - even when I got them over 3 years ago. I balance to within 10mV per cell at around 4V every month or so. I don't balance during every charge cycle.
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Unread 11-22-2019, 10:26 AM   #42
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Default Re: first step in DS conversion to lithium seems easy

I've ordered a multimeter with two decimal places for DC voltage, and I'll use your suggestion of a headlamp bulb to lower the voltage of each cell individually to 4.00V. I'm guessing I won't see much performance improvement, just longer battery life?
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Unread 11-22-2019, 11:08 AM   #43
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Default Re: first step in DS conversion to lithium seems easy

Balancing doesn't show up as a performance improvement but it does minimize issues with charging / discharging. I "Top Balance" my pack by balancing up near full charge. (4V or higher) If I do that when I discharge the pack down to around 20%, the balance isn't quite as good but it's dead on when nearing full charge during charging. That means no cells get overcharged. (My BMS would prevent that by monitoring all cell voltages) I never discharge below about 20% SOC. I don't need to because I have enough range to play at least 3 rounds of golf on a charge.
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Unread 11-22-2019, 01:25 PM   #44
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Default Re: first step in DS conversion to lithium seems easy

Quote:
Originally Posted by Volt_Ampere View Post
...I never discharge below about 20% SOC. I don't need to because I have enough range to play at least 3 rounds of golf on a charge.
What is 20% SOC? Is it 20% of the range between whatever you choose to charge to, and whatever you set as your lower voltage limit? So if I charge to 48.5V and discharge to 40.0V, then 20% SOC would be:

(48.5V - 40.0V) * 0.2 + 40.0V = 41.7V
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Unread 11-22-2019, 01:32 PM   #45
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Default Re: first step in DS conversion to lithium seems easy

NO! SOC is State of Charge. 20% means you have 20% of the total capacity left. Voltage is NOT linear with SOC. Not even close to linear. You can only roughly predict SOC by voltage.
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Unread 12-05-2019, 02:31 PM   #46
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Default Re: first step in DS conversion to lithium seems easy

Not much progress lately, what with Thanksgiving and all. I did order a battery state of charge gauge from Renology on Black Friday, which arrived yesterday. Hopefully I'll get it in next week, after being out of town again this coming weekend. I also have a second Chevy Volt battery coming from eBay, so I should get more range. So my tasks are:
  • top-balance existing battery
  • install SOC meter and shunt
  • install new battery

I did finish the wrap before I left for Thanksgiving. It is... adequate. Looks good from 10 feet away. I think if I did three or four more I would get the hang of it. I'll leave it on for now, but eventually I think I wanted it painted. Or just a new body.
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Unread 12-10-2019, 08:42 AM   #47
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Default Re: first step in DS conversion to lithium seems easy

Some progress last night. I got the second battery mounted. It came without threaded rods to hold the cells together, so I had to cut and bolt rods. I reconfigured the wiring. I thought this would be the final configuration, but I can already see things I need to change, so I haven't spent the time to cut and route it to look neat.

I rechecked the individual cell voltages on the first battery. Previously I was going through the BMS cable and saw a lot of mismatches, but when I put the leads right on the battery terminals all of the cells were at 3.79V, so good news there.

I need to find a way to connect both batteries to the shunt for the state of charge meter, so the SOC meter is not in.

I want to lower the max voltage on my new charger from 50.4V to 49V, so I opened up the new charger. There are five trim pots in there. One must be max voltage, one might be float voltage, but no idea on the other ones. Somehow I don't think that putting a load and a meter on there and randomly turning pots is a good idea.

I'm waiting on two new cables to connect in the second battery. I've read that the two batteries will level themselves, so they don't have to be at the same voltage when I put the charger on them in parallel? Is that correct, or should I try to get them close?
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Unread 12-10-2019, 09:06 AM   #48
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Default Re: first step in DS conversion to lithium seems easy

So you got a second BMS as well right?

The voltage output adjustment trimpot is usually the closest to the end of the DC power output side of the board.

Take a good picture indicating the trimpot position before beginning, one of them controls the max power and the other one the ramping down threshold voltage.

A load is not necessary to adjust the charger voltage since the circuit should regulate the output voltage.
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Unread 12-10-2019, 09:44 AM   #49
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Default Re: first step in DS conversion to lithium seems easy

Unless you parallel all of the cells you need to treat each battery separately with it's own BMS. Yes - they will have equal total voltage if you just parallel them but the individual cells will vary.
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Unread 12-10-2019, 11:03 AM   #50
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Default Re: first step in DS conversion to lithium seems easy

No second BMS yet. I don't need the low-voltage cutoff functionality because the Alltrax is set to cut off at 40V. And so far I've been manually monitoring the charging voltage so I don't get an overcharge. But the fact that all the cells are balanced to 0.01V makes me wonder if the BMS is doing that, or if the battery has just maintained the balance that it was shipped to me with?

Still, I suppose it is better to have the BMS. I've added a positive/negative connection block to the layout, since I was trying to stack up to five connections on the battery terminal, and that wasn't working. However, I don't see any way around having a connection for the BMS on at least the battery negative terminals. I've drawn my proposed BMS hookup below. Is there any way to keep just a single connection to all battery terminals?

I'll get better photos of the charger trimpots.
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