Lead acid voltage sage

5550 Views 67 Replies 7 Participants Last post by john61ct, Jun 29, 2017

9852

Discussion Starter · Jun 19, 2017

Hi, as I've stated my new to me EV ranger uses 19 Trojan t875 batteries for 152 volts. What I have noticed while driving is some pretty heavy voltage sage as the batteries get used. Is there any way to control the voltage sage, or should I just keep my foot off of the excelerator? It's not a huge deal I'm just wondering if there is any thing I can do about it.
Thanks

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Ampster

· #2 · Jun 19, 2017

As far,as I know the only thing you can do about with your existing configuration is to use less current, ie less accelerator. I am sure others will chime in about how Lithium Ion batteries tend to have less voltage sag.

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john61ct

· #3 · Jun 19, 2017

Yes, one of the huge advantages of LFP is a very flat V curve, even at high discharge rates.

A bigger bank may help a bit, but I think that's the nature of the beast with all the lead chemistries

9852

Discussion Starter · #4 · Jun 19, 2017

Yea that's about what I thought. Well I guess I'll just use less accelerator, that will help.
So just like a gangster I'll go Slow and Low.

Sunking

· #5 · Jun 19, 2017 (Edited)

Assuming your connections are good and cable sized properly, there is nothing you can do about it. It is just a function of the current and battery internal resistance.

Very familiar with Trojan batteries and the T-875 is a rather smallish 8 volt battery and when new fully charged has an Ri of roughly .005 Ohms so with 19 in series you have at least .1 Ohms of internal resistance plus any cable and connector resistance on top of that. The battery was designed for a 50 amp golf cart motor load. So at 50 amps you can expect at least a 5 volt sag, plus any cable and connector losses.

Typically Pb battery internal resistance is lower than lithium. However Pb batteries are a chemical reaction and not an Ion exchange. Thus a chemical reaction is slower and PB batteries cannot keep up with heavy discharges.

9852

Discussion Starter · #6 · Jun 19, 2017

Ekthor,
Ok lots of info, so I'll do my best to answer all of your questions.
1st ev specs:
94 Ford ranger, original curb weight was around 3200 pounds.
Ev curb weight is estimated at about the same, based on the weight of the truck minus the ice and all its components, plus the total weight of the batteries plus motor and transmission weight.
Electric motor: adc fb1 4001
Controller: Curtis 1231c 144 volts 500 amps
Batteries: 19 Trojan t875
100% charged =160 volts
I only see the real bag sag when I'm using a lot of current fast.
Full throttle current is about 375 amps for 20 seconds or so before I let off if I'm driving it hard.

9852

Discussion Starter · #7 · Jun 19, 2017

Ekthor, I agree about full charge voltage with my battery pack. I see 160 volts at full charge, and my meter is a chennic battery monitor, its set up to read 100% charged voltage at 160 volts, and 80% discharged at 149 volts, but if I'm really getting on it, the voltage can drop all the way down to 120, like I said it's not really a problem, because most of the time I'm driving to conserve power not eat it, its not a race truck. But today I just drove it like it was my f150 and noticed the sag.

9852

Discussion Starter · #8 · Jun 19, 2017

Ekthor,
No I did not keep the clutch. It works just fine without it.
I can jump it out on the highway and do 70 for a few miles
If I drive it easy, I get about 48 miles per charge. Normal driving about 35 miles.

9852

Discussion Starter · #9 · Jun 19, 2017

I just posted pics and specs in the garage

Sunking

· #10 · Jun 19, 2017 (Edited)

billhac said:
Ekthor, I agree about full charge voltage with my battery pack. I see 160 volts at full charge, and my meter is a chennic battery monitor, its set up to read 100% charged voltage at 160 volts, and 80% discharged at 149 volts, but if I'm really getting on it, the voltage can drop all the way down to 120, like I said it's not really a problem, because most of the time I'm driving to conserve power not eat it, its not a race truck. But today I just drove it like it was my f150 and noticed the sag.

You cannot use a meter to determine SOC on a Pb battery. Completely useless, only a hydrometer can give you real time SOC on a working system. Voltage can only be used on a well rested open circuit PB battery.

As for charge voltage on the T-875 you should be charging to 188 volts. 160 volts is even lower than FLOAT Voltage @ 171 volts. Where are you coming up with 160 volts? You charge at 188 volts @ C/10 to C/8 (17 to 21 amps) until charge current tapers to 2 to 5 amps. Try looking at Trojans Specs and User Manual page 19. If I were you i would get a good battery hydrometer and see where you are really at. My bet is you need a good EQ charge at 205 volts until Specific Gravity is equal. You are risking loosing your batteries from chronic under charge.

Lastly as I stated earlier, Pb batteries cannot keep their voltage up under heavy loads like lithium can because it is a chemical reaction, and chemical reactions take time and does not happen at the speed of light like an Ion exchange in a lithium battery. If you are seeing 120 volts under load, you have a serious problem. 133 volts (1.75 vpc) under load would be completely discharged. Anything under 145 volts is serous. You need to figure out what is wrong before it is to late. FWIW never take the batteries below 50%.

Good luck.

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9852

Discussion Starter · #11 · Jun 19, 2017

Sunking, I got the specs from the Trojan manual. It plainly states no load voltage is 8.48 volts x 19 batteries, you can do the math, that's resting voltage, so after the charge and not in use, so I had my battery monitor set for 160 for a full charge, and I don't know where you get to only discharge to 50 %, Trojan specs say 80% discharge is safe. As I said it really only happens at extreamy high current draw, if I use less accelerator I stay around 150. 188 volts is charge voltage, not full charge voltage, as soon as you turn off the charger that voltage will go down.

9852

Discussion Starter · #12 · Jun 19, 2017

My charger charges at about 220 volts at about 20 amps, I know I'm getting a full charge, it was just strange because I didn't remember getting that much voltage sag when I brought it home at 120 volts. Maybe I have a bad battery but It don't feel like its losing any power to quickly

9852

Discussion Starter · #13 · Jun 19, 2017

Btw, I did not use the batter monitor to determine the charge voltage, I use my fluke meter, then as I stated I had the monitor set up to read the voltage I got from my fluke, it works just fine.

9852

Discussion Starter · #14 · Jun 19, 2017

Go back and read some of my other threads, I have already proven you don't have to spend a ton of money to have a great ev, this is not my first build, so I do know how to determine proper charge voltage.i was singly wondering if anyone else had this problem and if the found a trick for it aside from using less current

9852

Discussion Starter · #15 · Jun 19, 2017

Well I just added water, but I have to admit I did not check voltages on each battery, but that is next, theses batteries are 7 years old so it's possible that they are going to start heading down hill on me

9852

Discussion Starter · #16 · Jun 19, 2017

Yea man, I have the original sales receipt for the batteries, and all the date codes are the same. So they have had a good long run. However, I think the truck was converted and not driven much, because the batteries were strong when I brought it home. It was originally run at 120 volts, but I bought 4 more batteries for it, and every since then it just seems like it does not have the same range, you would think it would get a couple more miles range but not really. It's got to be one of the batteries I bought for it. I really haven't had the time to look at it real close yet.

9852

Discussion Starter · #17 · Jun 19, 2017

Now I know someone is going to mention the fact that I'm running the Curtis controller at 152 volts nominal, but before I added that 4th battery to go up to 152, I did place a call to Curtis's service company to find out what the over voltage cut off was on the controller and explained to them what I wanted to do, the tech I spoke to confirmed that the over voltage cut off was 180 volts, so UNOFFICIALLY he said I could run it all the was up to 170 volts to be safe, as long as my controller was never on during the charging process. And I have a Spare controller just in case so I said what the hell, lets see what happens

john61ct

· #18 · Jun 19, 2017

billhac said:
these batteries are 7 years old so it's possible that they are going to start heading down hill on me

Most definitely, and pulling down to 20% will lose you half the lifetime cycles compared to 50%.

9852

Discussion Starter · #19 · Jun 19, 2017

Yea, it doesn't really go into overvoltage cut off to much in the manual, it says it has overvoltage cut off, but not what that voltage is. I'm having a bit of a heating issue myself, but I've been running my controller at 152 for weeks now without a problem, so I'm chalking my heat issue up to me running the truck hard and it being 110 degrees today, I'm going to cut back on the accelerator and see if that helps mine any. If not I'll be doing the same

9852

Discussion Starter · #20 · Jun 19, 2017

Ok, it's 117 degrees right now here in Phoenix, I just drove my truck around the block, before I took off I checked the controller temperature it was 101 degrees, just sitting there, just as I got in the driveway I could hear the overtemp alarm going off again, my conclusion:
ITS JUST TO DAMN HOT TO DRIVE THE EV. I'll have to start a thread on water looking a Curtis controller

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