Prius 12 volt charging current

I had the factory battery go dead while I was away traveling for 3 weeks 7 months after I bought the car. 6 months later I was gone 2 weeks and it was dead. I hadn't realized I needed to buy a charger and park within reach of a plug not having had a problem with previous cars. This caused me to spend time reading about the Prius 12v system and learning that the don't charge over 5 amp label on the battery is ignored by Toyota. The car is more than happy to supply as much charging current at about 14.4 volts as the battery will draw. It relies on the vent tube to keep gases out of the car. The battery is sealed so you can't add water. My 12v battery drained flat twice is already damaged as evidenced in reduced EV mode range. I also intend to camp in my Prius like it is an RV and run a 12 volt refrigerator. The factory battery is woefully inadequate for running a refrigerator for long periods of time without running the car. For both reasons I need more battery capacity and deep discharge design. I have replaced the 45 AH factory battery which came in my 2012 Plug-in Prius with a larger group 25 automotive lithium 12 volt battery (I will also be adding another battery in parallel and 300 watts of solar panels when camping with refrigerator).

I wanted to see what the charging behavior would be compared with what is documented elsewhere on there boards for lead acid batteries in earlier Prius models.

I used an old Heathkit Automotive meter which has a 90 amp shunt for measuring charge current. I rigged the shunt in the positive cable at the battery. I also used a DC current clamp on meter which I clamped over the negative battery cable. To test the setup I created about a 15 amp load and compared the two with a digital meter which has an internal 20 amp shunt good for measuring current for 15 seconds. All 3 where pretty close at 15 amps current draw.

I discharged the battery about 60%-70% and powered on the car.
Initial charge current measured on the shunt as soon as I got around to the back to read the Heathkit automotive meter. Result 60 amps. Clamp on meter 70 amps
After 5 minutes automotive shunt 47 Amps. Clamp on meter 60 amps
After 10 minutes automotive shunt 45 Amps. Clamp on meter 58 amps
After 15 minutes automotive shunt 44 Amps. Clamp on meter 57 amps
After 20 minutes automotive shunt 43 Amps. Clamp on meter 56 amps
After 25 minutes automotive shunt 43 Amps. Clamp on meter 55 amps
After 30 minutes automotive shunt 41 Amps. Clamp on meter 53 amps
After 35 minutes automotive shunt 41 Amps. Clamp on meter 52 amps
After 40 minutes automotive shunt 40 Amps. Clamp on meter 51 amps
After 45 minutes automotive shunt 37 Amps. Clamp on meter 49 amps
After 50 minutes automotive shunt 36 Amps. Clamp on meter 46 amps
After 55 minutes automotive shunt 27 Amps. Clamp on meter 36 amps
After 60 minutes automotive shunt 1 Amps. Clamp on meter 1.3 amps
After 65 minutes automotive shunt 0 Amps. Clamp on meter 0.1 amps

 

64 AH

 

Yeah. I have the Plug- in Prius and should see at least 12 miles EV only range indicated on the dash on a full charge. Been a long time since I saw that. 9-10 is all I have seen for a long time. The EV traction battery charges the 12 volt battery via DC to DC converter. The poor state of the lead acid battery caused it to draw more charging current than normal. That power is diverted from EV use to battery charging. I gained 2 miles of EV range as indicated on the dash display by replacing the 12 volt battery.

 

I agree. I didn't intend to imply otherwise. Just explaining my reasons for my battery choice.

 

There is a lot of cleaver engineering in the car. I assumed based on the label on the battery that they might be limiting the charging current delivered to the battery while the DC to DC converter supplies all of the cars 12v current needs once the car is powered on. I believe the DC to DC converter can supply 125 amps based on the label where if feeds into the main fuse block under the hood. The Prius battery is unique in dimensions, vent tube, pencil posts and this label to limit charging current. Why? Few manufacturers make a direct replacement battery. Toyota, Optima and Exide are the only ones I know of. There are many posts on this board mentioning the low charge current specified on the label of the factory supplied battery. It is actually the manual that says 5 amp. The battery says charge at no more than 4.2 amp as seen in the attached pic of the factory supplied battery that came in my car. Apparently Toyota thinks that people will go buy a low current charger and use it rather than the car to charge the battery before they drive the car if it gets heavy discharged. As long as it stays nearly charged from regular use the car won't charge it at too high a current and most people will get excellent long battery life. Parked at the airport far from a plug for 2 or 3 weeks falls outside of their parameters. If you have to jump it and drive home its going to be charged at over 10 times the labeled rate on the battery while you drive. They keep you safe by venting all the gassing to the outside.

 

Then there is what it says in the owners manual about jump starting and charging to consider. Perhaps the truth lies somewhere in between. No matter. All I intended to do is share the actual charging behavior of the car for those interested and those considering replacing their batteries with indirect replacements.

 

There are bits about the 12V battery in several place in the manual. Page 472 Warning about it producing hydrogen gas. Disconnect the negative cable before charging the battery while in the vehicle. Page 474"How to charge the battery" " Only perform a slow charge (5 A or less). The 12 Volt battery may explode if charged at a quicker rate" So why does the car charge a 8 - 10 times that rate if it may explode? Glad I don't have that charge rate sensitive explosive battery in the car any more. Page 594 - 596 covers jump starting pretty much as you boiled it down except the donor car is supposed to be running at elevated engine speed for that 5 min before you try to start the Prius. It also states that once vehicle is started to have the vehicle inspected at a Toyota dealer as soon as possible. Maybe I should have the Dealer inspect the charge rate. Maybe the high charge rate I see is a malfunction though I tend to doubt it.

 

I don't believe you have addressed Toyota asserting in the owners manual Page 474"How to charge the battery" " Only perform a slow charge (5 A or less). The 12 Volt battery may explode if charged at a quicker rate" And the label on the battery itself "never charge quickly" specifying 4.2 amps. I don't see a difference between the car charging at a higher rate than this or using a separate battery charger and charging at a higher rate. Your calling my actual measurements of the cars charging current a "little theory" is uncalled for.