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Old 06-19-2017, 12:21 PM
9852 9852 is offline
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Default Lead acid voltage sage

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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  #2  
Old 06-19-2017, 12:44 PM
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Default Re: Lead acid voltage sage

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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Old 06-19-2017, 01:30 PM
john61ct john61ct is offline
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Default Re: Lead acid voltage sage

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
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Old 06-19-2017, 01:36 PM
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Default Re: Lead acid voltage sage

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.
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Old 06-19-2017, 01:46 PM
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Default Re: Lead acid voltage sage

Hello.

Voltage SAG is unavoidable and most evident in FLA (PbA) batteries compared to LiFePO4 cells at both charging and discharging processes. The more amperage you get from the battery pack, the more SAG you will see. You may shorten their life if you get too much amperage from them.

Let's share experience.

1st of all, I'm using almost exactly the same voltage you use. I use 13 x 12v FLA 120Ah LTH batteries for a total of 156 Nominal Volts in a classic VW beetle. Im adding "only" 351 Kg of batteries in a 18.7 KW pack , whereas you are adding 551Kgs of Lead Acid Batteries in a 25.8 KW pack. You have spent more $ and will spend more watts per mile.

Here my components
Motor: ADC 9"
Controller: Kelly KDHE144V600A
JLD404 as a nice meter.
My car's weight is around 1,100kg.

*** Please share your specs: motor, controller model, car's weight (at least estimated based on your car's model)


At full charge I see 170VDC in my battery pack, far above from the nominal 156V.

What is your voltage reading at full charge, open circuit? If you do the math, you may say my batteries are overcharged... well not precisely, trojan says that a proper reading must be after 6 hours of charge, (best if read after 24h)

"For accurate voltage readings, batteries must remain idle at least 6 hours, preferably up to 24 hours."

For EV users, this idle time is near to impossible. So the over voltage is virtual, irrelevant as long as the charger ends at the specified 100% SOC.

Going back to Voltage SAG:

***What are your readings at full throttle? I mean
How many AMPS you read from the battery pack to the controller?
What is the voltage reading at full throttle point?
Optionally:
What is the amperage you see from your controller to your motor?
What is the voltage you see at the same point?


My answers:
I get 200 Amps for a few seconds only. Remember Kelly has a bad reputation in their controllers.

Climbing long hills at 100Amps somewhat constant, I can see 150Volts in my pack, so I am not hurting the batteries.

As I said, you have to be careful of how many amps you get from Flooded Lead Acid batteries, it depends on each case (6V usually rated at 230Ah let you get more amps, but you need more Heavy Lead)

On one side we have Trojan, they say in their data sheet, when they say your 8V has a 170Ah capacity:

"The amount of amp-hours (AH) a battery can deliver when discharged at a constant rate at 80F (27C) and maintain a voltage
above 1.75 V/cell. Capacities are based on peak performance. "

*** For 8V batteries you better keep voltage ABOVE 1.75vpc x 4 = 7V x 19 = 133Volts, even if your controller can get more amps, 133V should be your bottom voltage (SAG), if you need to get more amps, do it for a few seconds only.

Now if you have a programmable controller, you can set that bottom voltage configured in the controller and it will protect the batteries for you, it may alert you or simply will stop until the voltage recovers naturally.

On the other side we have NetGain guys say in their manual:

"EV racers found it necessary to build 336 Volt + battery packs due to the fact that 12 Volt PbA batteries would sag to 5.5 Volts when 1,400 Amps were drawn from them. If the motors are capable of handling 168 Volts, and the PbA battery pack is 168 Volts nominal, then the motor would only see about 77 Volts when 1,400 was drawn from the battery pack. In order to deliver the maximum voltage to the motor, it became necessary increase the PbA battery pack voltage. As LiFe battery voltage typically sags far less under heavy amperage draws, the battery pack voltage need not be as high as a comparable PbA battery pack voltage."

This is the reason why most of the EV community discard PbA, but if you respect them they will last years.

As I said, please share your experience and components' specs.

How many miles you get with your EV? What is their max speed?

Regards,

Hector
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Old 06-19-2017, 01:52 PM
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Default Re: Lead acid voltage sage

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.
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Old 06-19-2017, 02:27 PM
9852 9852 is offline
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Default Re: Lead acid voltage sage

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.
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Old 06-19-2017, 02:36 PM
9852 9852 is offline
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Default Re: Lead acid voltage sage

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.
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Old 06-19-2017, 02:48 PM
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Default Re: Lead acid voltage sage

Hi billhac,

Did you retain the clutch?

In my VW without it was very hard to shift gears.

What is the maxspeed you get in the truck?

How many miles can you get?

We are using the same motor.

That Curtis is not configurable, so my suggestion is to keep an eye on your voltage sag to avoid going down beyond the bottom.

Regards
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Old 06-19-2017, 03:00 PM
9852 9852 is offline
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Default Re: Lead acid voltage sage

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.
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