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How long does your 12V auxiliary battery last?

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Hi all,

I use a 22ah deep cycle auxiliary battery for my electric MR2 and after close to 2 years of driving, it is showing signs of degradation. After parking 3 days over a long weekend without driving, the ignition fails to turn on.

I've recently swapped it out with a new one, and took the opportunity to measure the amp draw of my 12V system.

http://blog.mr2ev.com/how-to-size-the-12v-battery-for-an-electric-car/

Please share the following:
How long does your 12V batteries last?
How long can you park your car?
What's your parked amp draw from the 12V system?
Do you use a BMS?

Thanks!

Ken
http://mr2ev.com
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Discussion Starter · #3 ·
Or just install a dc/dc converter like everybody else :)
TEV, thanks for your response. I guess I didn't make it clear that I do have a DC-DC converter. It is only turned on when the ignition is on. Do you mean to keep the DC-DC converter on at all times? Is that what folks typically do? I guess I am worried that if I were to ever leave the lights on when I am at work, it could drain the main pack and affect my range to get back home.

Ken
 

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I put in a switch to disconnect the 12V battery from all loads if I'm not going to be using it for a few days. I actually have three switches, the main circuit breaker for the main pack, a switch on the output of the DC/DC converter, (it has an LED that stays lit and could drain the 12V if not shut off), and the switch for the 12V battery. All are shut off for long term storage periods.
 

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Hi all,

I use a 22ah deep cycle auxiliary battery for my electric MR2 and after close to 2 years of driving, it is showing signs of degradation.
Two years is about the average life for a lead acid depending upon the duty cycle and level of maintenance you pull--do you keep the electrolyte topped up, ever over charge or discharge?, etc...
 

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I think it depends a lot on the temperatures where you live. Here in NY I have starting batteries lasting between 5-10 years. As for maintenance, he is using a sealed AGM battery, no maintenance. I think in an EV that doesn't require high cranking loads or deep discharges the battery should last much longer than two years.
 

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I've recently swapped it out with a new one, and took the opportunity to measure the amp draw of my 12V system.

http://blog.mr2ev.com/how-to-size-the-12v-battery-for-an-electric-car/

Please share the following:
How long does your 12V batteries last?
How long can you park your car?
What's your parked amp draw from the 12V system?
Do you use a BMS?

Thanks!

Ken
http://mr2ev.com
How long does your 12V battery last? you mean calendar life? I am using LiFePO4 cells for my 12v battery. I expect them to last as long as everything else. At least 8 years. It is an 8AH battery made up of the 2.3AH A123 cells in a 4S4P arrangement. The DC-DC is 40amp and it takes very little time with the DC-DC on to recharge.

I've parked it 2 weeks without issue.

The car draws about 8ma which is the clock and radio preset maint only. Would be drained too low to turn on the DC-DC converter relay at around 1000 hours of sitting. This is about 42 days. Thinking of getting rid of the car's clock but I think it draws almost nothing.

No BMS is needed with LiFePO4 type cells if you know what you are doing. If you don't know what you are doing a BMS is probably not going to help you. The odd thing is the people who could get the most use from a BMS are those who need them the least.
 

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I am using the same FLA 12V accessory battery that was in the car when I purchased it for conversion. I've had the car almost 4 years, bit over 3 1/2 as an EV. Never had any problem with the 12V battery. It is a Les Schwab 60 month. Unfortunately they didn't mark the date/year of purchase. Earliest year tab on the battery is 2006. It looked fairly new when I purchased the car, so was maybe 1 - 2 years old. The DC/DC is wired to the main pack through an ignition switch controlled relay like yours. Output wired directly to the 12V battery. BMS is the minibms. I have left the car sit for up to 12 days, no problem. Temperatures here typically range from around 100 F in summer to 5 F in winter.
 

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Hi, my dc/dc is on only when the ignition is on , I am using the battery I got when I got the car before conversion it was allready not a very good battery when I start using it in EV mode. Any car in long storage will drain the battery, if is not disconnected and a old/bad battery will self discharge too.

Also I think that the life of a battery in an EV will depend very much of the dc/dc being able to keep up with the power needed to run everything and not discharging the battery very much/often.
 

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Discussion Starter · #10 ·
Two years is about the average life for a lead acid depending upon the duty cycle and level of maintenance you pull--do you keep the electrolyte topped up, ever over charge or discharge?, etc...
kennybobby,
I use a deep cycle sealed lead acid 12V battery, so there is really no maintanence. The 12V battery is connected to an AstroDyne SD-350 DC-DC converter to keep it charged when the ignition is turned on. I am not sure how the DC-DC converter controls how to prevent overcharge?
 

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Discussion Starter · #11 ·
How long does your 12V battery last? you mean calendar life? I am using LiFePO4 cells for my 12v battery. I expect them to last as long as everything else. At least 8 years. It is an 8AH battery made up of the 2.3AH A123 cells in a 4S4P arrangement. The DC-DC is 40amp and it takes very little time with the DC-DC on to recharge.

I've parked it 2 weeks without issue.

The car draws about 8ma which is the clock and radio preset maint only. Would be drained too low to turn on the DC-DC converter relay at around 1000 hours of sitting. This is about 42 days. Thinking of getting rid of the car's clock but I think it draws almost nothing.

No BMS is needed with LiFePO4 type cells if you know what you are doing. If you don't know what you are doing a BMS is probably not going to help you. The odd thing is the people who could get the most use from a BMS are those who need them the least.

dougingraham,
8mA draw is great compared to my 85mA draw, I wonder if the DC-DC converter is drawing from the battery? I need to check that out and report back. I've heard from others that it has happened to them. Do you have a SOC gauge? My EV display 2 works really well, it gives me a sense of how much charge I have left so I can plan ahead, drive slower, find a charging station, etc, but it draws from the 12V constantly.

As for BMS, I do not use one either, I use a set of cell log 8's to measure all the 45 LiFePO4 cells in less than 2 minutes.
I was just wondering what the 12V amp draws is from those that do have BMS installed.

Thanks for sharing!
 

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Discussion Starter · #12 ·
I am using the same FLA 12V accessory battery that was in the car when I purchased it for conversion. I've had the car almost 4 years, bit over 3 1/2 as an EV. Never had any problem with the 12V battery. It is a Les Schwab 60 month. Unfortunately they didn't mark the date/year of purchase. Earliest year tab on the battery is 2006. It looked fairly new when I purchased the car, so was maybe 1 - 2 years old. The DC/DC is wired to the main pack through an ignition switch controlled relay like yours. Output wired directly to the 12V battery. BMS is the minibms. I have left the car sit for up to 12 days, no problem. Temperatures here typically range from around 100 F in summer to 5 F in winter.
tomofreno,
Thanks for the data point, your amp draw sounds like it is much lower than mine even with a BMS. Do you use an SOC gauge like the EV display 2?

Ken
 

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kennybobby,
I use a deep cycle sealed lead acid 12V battery, so there is really no maintanence. The 12V battery is connected to an AstroDyne SD-350 DC-DC converter to keep it charged when the ignition is turned on. I am not sure how the DC-DC converter controls how to prevent overcharge?
What voltage is it set at, is it adjustable?
 

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You need to be careful with that since the DC/DC will put out a constant voltage, not a charge cure. For example to properly charge the Odyssey AGM battery I'm using the charge curve is to take it up to 14.8V when charging but to float it at 13.2-13.8. If I just set the DC/DC to 14.8 then it will probably kill the battery eventually. I run mine in the float range and then occasionally charge it to 14.8V.
 

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Discussion Starter · #16 · (Edited)
Ken,

After looking at the spec sheet of your dc-dc, the output is adjustable. Find out what is the charge voltage of your 12V lead acid battery and adjust the output of the dc-dc to fully charge it every time you key switch to turn on the dc-dc.
Honn1002, JRP3,

With my ignition turned on, the voltage across the battery terminal is 13.39V, which should be sufficient to charge the battery. Standing voltage for a fully charged battery is 12.9.
 

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Re: Security Alarm?

If you remove all 12 volt power sources (battery and DC-DC convertor), you can measure resistance across the 12 volt + and - wires. Using V=IR, at 13.5 V and 85 mA, you would expect to measure 160 Ohms impeadance. Pull fuses one-by-one to find the branch circuit that is drawing the current. I would expect at least 1,000 Ohm with the car powered down.
 
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