# How long does your 12V auxiliary battery last?



## kchiangusa (Mar 28, 2012)

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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## TEV (Nov 25, 2011)

Or just install a dc/dc converter like everybody else


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## kchiangusa (Mar 28, 2012)

TEV said:


> 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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## JRP3 (Mar 7, 2008)

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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## kennybobby (Aug 10, 2012)

kchiangusa said:


> 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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## JRP3 (Mar 7, 2008)

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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## dougingraham (Jul 26, 2011)

kchiangusa said:


> 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/
> 
> ...


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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## tomofreno (Mar 3, 2009)

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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## TEV (Nov 25, 2011)

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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## kchiangusa (Mar 28, 2012)

kennybobby said:


> 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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## kchiangusa (Mar 28, 2012)

dougingraham said:


> 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.
> 
> ...



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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## kchiangusa (Mar 28, 2012)

tomofreno said:


> 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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## JRP3 (Mar 7, 2008)

kchiangusa said:


> 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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## honn1002 (Nov 26, 2008)

JRP3 said:


> What voltage is it set at, is it adjustable?


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.


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## JRP3 (Mar 7, 2008)

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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## kchiangusa (Mar 28, 2012)

honn1002 said:


> 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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## kennybobby (Aug 10, 2012)

*Security Alarm?*

Do you have an alarm system on there--last time i saw 'off' currents that high was on a broken alarm system that was draining an ICE customer's battery.


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## Joey (Oct 12, 2007)

*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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## tomofreno (Mar 3, 2009)

kchiangusa said:


> 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


 Yes. TBS ExpertPro. I think you may be overcharging your sealed battery.


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## kchiangusa (Mar 28, 2012)

*Re: Security Alarm?*



kennybobby said:


> Do you have an alarm system on there--last time i saw 'off' currents that high was on a broken alarm system that was draining an ICE customer's battery.


I do have an alarm system and an aftermarket power door lock system. What's been a typical amp draw for your customer? Thanks!


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## kchiangusa (Mar 28, 2012)

tomofreno said:


> Yes. TBS ExpertPro. I think you may be overcharging your sealed battery.


tomofreno,
My 12V battery's fully charged standing voltage is 12.9V and when I turned on the ignition (i.e. DC-DC on), the voltage is 13.39V. I think the DC-DC is configured to slow charge the 12V battery. I drive a 1/2 hour daily to and from work. Do you still think I may be overcharging?

Thanks!

Ken


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## honn1002 (Nov 26, 2008)

kchiangusa said:


> 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.


Ken,

13.39V with the dc-dc on is too low to charge your 12V battery. Your 12V is not properly charged to it fullest capacity. Here is a link the tempest battery you're using http://www.tempestbatteries.com/html/td22-12.html

Cycle charge is 14.4 to 14.7V
Float charge is 13.5 to 13.8V

Your charge voltage of your dc-dc is below the float charge. I think your 12V battery only gets recharged to around 50% or less of its 22Ah capacity. This is based purely on my speculation. 

To experiment, use your rc hobby charger and set the charge curve for lead acid battery. Disconnect your 12V battery from the car and charge it with your rc hobby charger. You will find out how many more amps flowing to the 12V battery. 

Honn


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## kchiangusa (Mar 28, 2012)

honn1002 said:


> Ken,
> 
> 13.39V with the dc-dc on is too low to charge your 12V battery. Your 12V is not properly charged to it fullest capacity. Here is a link the tempest battery you're using http://www.tempestbatteries.com/html/td22-12.html
> 
> ...


Honn,
You are right, the DC/DC converter is set too low for my battery. Just added another 4AH to my battery using the RC charger. I've also adjusted the charge voltage from the DC to DC to 13.75. 

Thanks for everyone's suggestions! I will add a periodic maintenance item to fully charge the 12V battery using my RC charger. 

How often do you guys fully charge the 12V battery to 14.8 volts?


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## dtbaker (Jan 5, 2008)

I have built both my EVs with 'always on' dc-dc and a *small* (4ah) ATV sealed lead starter battery in parallel. dc-dc puts out 13.8 and battery soaks up the occasional sag/spike when things like vac pump and headlights are turned on and off.

If the vehicle(s) sit for more than 5 days I charge to top things up, or if I know I won't be driving for more than 5 days I disconnect both main and small aux12v to prevent drain.

I left for a week one time and it took something like 1kWhr to top off the pack to recover the drain from 12v system, dc-dc and controller on standby. So, I could go weeks without worry I suppose...


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## kchiangusa (Mar 28, 2012)

Thanks again for all the helpful suggestions. One more thing I wanted to wrap up. My connection from the DC/DC converter to the 12V battery has a relay on the 12V+ side (see schematic here). 

Therefore, my DC/DC converter is not contributing to the 85mA draw from the 12V battery.

With the changes of high float charge and the periodic maintenance charge, this new 12V battery should last a bit longer than 2 years I think.

Ken
http://mr2ev.com


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