My Lifepo4 12V battery setup

[Jiminy]

Hello all,

I wanted to show my 12V setup.

Long story: About 6 years ago, a friend and fellow Mitsubishi I-MiEV owner picked up a tiny lithium motorcycle battery at a garage sale. He bought it super cheap and didn't need it so he gave it to me since he knew I was into lithium batteries. I also didn't have a use for it at the time so I shelved it after making sure it had good storage voltage. A couple of years later the 12V battery died on my I-MiEV. After researching the cost for a new one I decided to put the 7Ah Shorai battery in there to see what would happen. I made some wires for it, hooked them up and what-do-ya-know, it worked! I glued it down with silicone rtv adhesive to the stock tray so it wouldn't move around. Worked fine for two years until I got bored with it. I bought a 20Ah battery from Amazon to replace it since I thought it would make a great substitute for lead acid for others. Since 7Ah would be hard sell I supersized it. I've been using the 20Ah battery for a couple of years now with no issues. Bear in mind I live in Texas. I also keep my car in a garage where it rarely gets below 50F. I even converted a Nissan Leaf and 500E I owned last year with perfect results.

SO, if you are adventurous and would like to have a 13.8 resting voltage on the 12V battery then this might be for you. If it gets below freezing where the car sits then you might devise an insulation package to keep it warm. I would just print a jacket for it.




This is actually a friend's car. I bought tapered brass battery terminals with 6mm studs so make the install very stock-like.

Proceed at your own risk but I will always use lithium batteries for my low voltage duty on an EV.

 

[Jiminy]

I designed a custom tray system to perfectly fit both the battery and the crossmember where it sits. This is my car with the tray installed:



And a rendering of the tray:

 

[Jiminy]

I found a picture of the 7Ah Shorai battery which I later sold on eBay for $70.

 

[turbogts22]

Great write up! When I upgrade mine I'm thinking of trying out a sodium ion battery. These work well down to -40F unlike lithium. I live in a northern climate with cold winters so lithium isn't an option.

 

[Jiminy]

I'll have to take a look at that.

Do you think it's possible to keep the lithium above freezing temp? I used to move my batteries inside the cabin for better weight distribution. Not necessary with a 5 pound battery. Seem like relocating it and using enough insulation would go far to keeping it happy. I have no concept of cold here in Texas

 

[turbogts22]

Our EV's are garage kept in the winter months so the brutal cold typically an issue. My concern would be if I travel anywhere and the car is left out. -9F the other morning was dang cold for this time of year. Aliexpress has a variety of sodium 12v battery sizes to choose from and the prices are slowly coming down.

 

[MickeyS70]

My concern would be if I travel anywhere and the car is left out.

Should you not also be concerned about the main HV battery?

 

[hjdlsnbc]

Looks to be unknown here based on the discussion above so i drop the information here:

To get a fully compatible replacement for the lead acid battery with its capabilities to get charged at low temperatures, you have to add Yttrium to the battery chemistry.
You really do not want the imiev dcdc converter that is sort of exploding with the amount of current in direction to the 12v battery, in combination with low temperatures and regular lifepo4 batteries. From technical point of view this is a bad combination. It also does not make that much sense when you can get the correct battery with Yttrium inside and then do not have to bother any more about low temperatures:
https://lypbattery.com/product-category/lifeypo4-starter-battery/
https://en.winston-battery.com/?qdxdc/

 

[turbogts22]

Should you not also be concerned about the main HV battery?

I'm not trying to charge the HV battery when it's well below freezing so that isn't the concern. The 12v gets charged whenever the car is on/ready and lithium 12v batteries don't always play nice with this depending on the chemistry. If I moved south none of this would even matter but that'll never happen.

 

[Jiminy]

Looks to be unknown here based on the discussion above so i drop the information here:

To get a fully compatible replacement for the lead acid battery with its capabilities to get charged at low temperatures, you have to add Yttrium to the battery chemistry.
You really do not want the imiev dcdc converter that is sort of exploding with the amount of current in direction to the 12v battery, in combination with low temperatures and regular lifepo4 batteries. From technical point of view this is a bad combination. It also does not make that much sense when you can get the correct battery with Yttrium inside and then do not have to bother any more about low temperatures:
https://lypbattery.com/product-category/lifeypo4-starter-battery/
https://en.winston-battery.com/?qdxdc/

But how much do these cost? A 50Ah lithium battery is significantly greater in capacity than the 34Ah stock lead acid battery. Possibly 3x greater in usable capacity. The 20Ah Chins I used cost $55 at the time I bought it. They are $70 now.

I live on planet Texas and the battery I have used is sufficient for my climate. It may not be for where you live. I posted this after 3-4 years experience with this chemistry.

 

[Jiminy]

I have also confirmed with an ammeter that the 12V battery is doing approximately nothing once the DC-DC kicks on when in Ready Mode. There is 0W going into the properly charged 12V battery. As you know, it gets topped up every time the HV battery gets charged.

 

[MickeyS70]

I'm not trying to charge the HV battery when it's well below freezing so that isn't the concern.

What about regen?

 

[Jiminy]

I want to apologize to hjdlsnbc for dismissing the Yttrium suggestion. I think it would be a great idea to have low temperature capability so that everyone could enjoy the benefits for getting away from lead acid low voltage batteries. I'd like to hear more from hjdlsnbc.

I posted this so that more people in temperate climates could see that a switch to a cheap garden variety Lifepo4 of no more than 20Ah is possible. I love the damn things since they are cheap, light and have higher resting voltage. The other thing I forgot to mention is that when I ran the 7Ah Shorai battery there were a couple of times when I did not use the car for more than 14 days. When I came back to start the car there was no 12v power. SO, I got out a 12v model airplane lipo battery and hooked it up to the 7Ah battery with very small alligator clips. This is a 2200 milliamp hour 3S lipo so it only has 12.6v fully charged. I am sure it wasn't fully charged. It probably had only 12v on it as I like to store lipos at 4v/cell. Anyways, I hooked it up to the system and it turned the BMS back on inside the Shorai. The Shorai BMS turns off output if the voltage drops too low. This is common on BMS controlled batteries. So, I unhooked the lipo and the car was able to start and then charge the Shorai again with the DC-DC. I never had to charge it with a charger. It just worked from then on. This happened twice. A little annoying?, why yes, but predictable and very easy to fix with a quick jump. This is the reason I wanted to put a much larger 20Ah battery in it's place as it would be able to go longer without running down to the lvc or low voltage cutoff. Since I switched to the Chin's I have done a few periods of not using the car for 14 days and the Chin's has never cut off. That makes sense since it has almost three times the storage as the Shorai.

More food for thought.

 

[JoeS]

Regarding the use of '12v' LiFePO4 batteries in i-MiEVs, it's important to note that we rely on the battery's built-in BMS to handle overvoltage, undervoltage, overcurrent, over-temperature, and under-temperature, of not only the entire pack but the individual cells as well. Generally speaking, this is well-understood and modern BMSs seem to do this admirably.

Of concern to me is the possible overvoltage (out of our i-MiEV dc-dc) situation which can show up at very low temperatures. We still do not have conclusive accurate voltage measurements in the sub-zero (°C) region.

This possibility raised its ugly head when I was evaluating various LiFePO4 batteries for use in the i-MiEV, as a number of manufacturers stress that the absolute maximum input voltage they want to see at the battery terminals is 14.60vdc (i.e., 3.65v/cell).

For example, this was a disqualifier in the Hyundai Kona Electric as their 12v dc-dc is designed to output 12.8vdc to feed the calcium maintenance-free lead-acid battery.

Some LiFePO4 manufacturers, such as OHMMU, have told me that is not a problem for them as their BMSs are designed to handle such a higher voltage.

For myself in my moderate climate my '12v' NOCO LiFePO4 8Ah battery is going into its third year completely trouble-free, and about all I do is NOT keep a bluetooth dongle permanently connected, DON'T sit in the car with the switch in ACC and the radio playing for more than a few minutes, and DO feed the 12v battery with a very small Mean Well 13.4vdc power supply whenever the car isn't going to be used for more than a couple of days.

Here's another thread on the topic:
https://myimiev.com/threads/12v-lifepo4-and-noco-battery-discussion.5450/page-2#post-47633

 

[Jiminy]

Interesting details Joe. On the Hyundai have you seen a limit of 12.8V when the car is in Ready mode? I can't believe Hyundai would do that and I am pretty sure they didn't barring hearing it from you. It should show in the low to mid 14's when charging. Lead batteries need a good differential to actually charge.

The Nissan Leaf, Fiat 500E, eGolf and the Mitsubishi will charge at up to 14.5V which is perfect for the Lifepo4 battery. I use the lead acid setting when I charge them up when brand new since they are never charged. It usually takes 3-4 hours at 2A with my iCharger to get them fully charged. After that I install them and that's it. I do regularly charge lead acid batteries every coupla months at a 2A rate with my iCharger since they are never fully charged by the DC-DC (my eGolf and my son's Gen 2 Prius). I do not worry about this or do this on the Lifepo4.

I'll bet it was a little leap of faith to run that 8Ah battery. Have you ever had it time out like the Shorai did for me? Like you, I never let any 12v battery run a vehicle for long without an alternator or DC-DC working. Like I mentioned before, when the DC-DC is engaged there has been no current flow into the 12V battery every time I have checked with an ammeter. I'm sure it happens but it is likely when charging the HV battery. The 12V just allows the the system to wake up and the DC-DC does all of the work powering the low voltage system from then on.

 

[Jiminy]

As an aside I looked for 20Ah Lifepo4 batteries on Amazon.fr and Amazon.uk but there were absolutely none. There were some 7ish Ah batteries but then it went up to 100Ah. Seems there is a protectionist mechanism or some weird UK/euro rule about it. Makes me kinda annoyed. The Chin's has plenty of company in the US Amazon. The price has gone up though. I want to buy one, cut it open and make a BOM to make my own from China sourced cells and BMS.