Recharging after Dead 12v Battery

In my observations, I have seen 14.4V put out by my Gen 2 DC/DC converter and this directly correlates to the battery being significantly low. When I personally see this for any significant time, I get the battery on a charger.

 

Am I the only one who finds it harder to figure out/repair the 12v auxiliary battery than the HV nimh 'scary' battery???

 

What do you find hard about it? (Genuine question)

Generally, there is nothing to repair. You charge it. If it won't hold a charge, you replace it. Pretty straight forward, I would have thought.

 

Yes, I was working on a Gen2 that had a key off draw that discharged the 12V down to about 6V. Charging the battery out in the parking lot wasn't an option, so I connected a booster pack, powered up the car and brought it into the shop. I was using an amp clamp to track down the draw, and decided to test the car's charging system while I was there.

With the booster pack still connected I readied up the car while measuring current from the battery to the car. It takes 20 to 25 amps to power up the car. Once the dc-dc converter turned on I had 80 amps going into the 12V and booster. Disconnecting the booster dropped current to 60 amps going into the battery. Voltage at the battery was 13.7V. I then shut it down and put an external charger on it.

Yes the Prius can do minor adjustments to system voltage, but I haven't seen anything outside of 13.7 to 14.2V. In order to limit current to the 12V you need to A) measure the current - I don't see any form of current sensor or shunt resistor between the 12V and rest of the car, and B) limit the voltage when required. When I connected my charger in constant current mode (6A) voltage was near 7V(? don't remember the exact amount) . As the state of charge (and internal resistance) slowly increased, voltage increased while amps were the same until it neared capacity. That's Ohms law in action V=IxR. (unless this battery is faulty? It load tested OK. But it's been gone for a couple months with no complaints). OK- thought about it some. The charge acceptance rate has to depend on both the internal resistance of the battery as well as the delta V (difference in the voltage level from the source to the battery). This particular case the delta V is very high but I would expect the resistance to also be high.?

Automotive charging systems are "constant" voltage, adjusting to maintain a preset voltage level. Many can make some adjustments for temperature compensation but will charge the same whether the battery is at 95% SOC, or 25% (and you jumped it). The only thing that can limit current is the battery's resistance.

Here's the scope capture from that event- the amp clamp accuracy was confirmed- This battery was taking many amps.

The point is that the car will supply as many amps (up to its maximum output) as the battery can accept.

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Gen 1 and Gen 2 have a skinny wire, "DC/DC S", that runs from the converter all the way back to the battery positive clamp assembly, in parallel with the heavy power wire, acting as a high-impedance voltage sensor of the voltage directly at the battery. That allows the converter to know the voltage right at the battery, and (as it certainly knows the voltage at its own output) possibly to use the somewhat-known small resistance of the power wiring as a crude shunt for an estimate of current. That of course is rougher than a calibrated shunt.

Gen 3 still has the DC/DC S input to the converter, but instead of running it all the way back to the battery, they use it to measure the underhood fuse box voltage. They may have decided that was good enough, and reduces the effective 'shunt' to be just the cable between the converter and the fuse box, so it's less able to isolate the current going just to the battery. At the same time, Gen 3 added a temperature sensor above the battery.

Gen 4 adds an all-singing, all-dancing battery state sensor assembly integrated with the negative battery clamp.

The current you measured greatly exceeds any that I have measured on a Gen 2 resurrected by a jump, but I may have been measuring on a less drained battery. When you say it was at 6 volts, that was not under load? That's very much deader than dead. A battery can still be unable to start the car, and need a jump, without being nearly that dead.

 

This was 6V unloaded, so it was dead, dead, pushing-up-the-daiseys, gone to the heavenly choir above, dead. But it got better.Again, I wasn't expecting that much draw from a discharged AGM but there it was.

Hmmm. An integrated battery health module. I have seen that. It sounds like what Honda has on some cars (the pic looks exactly the same), GM and Ford like to put Hall sensors on the battery cables. The ecm monitors the state of charge, when it gets mostly charged up it can drop the charging voltage setpoint down (12.6V on Honda) to save gas. Actually kind of surprised the (gen2) Prius doesn't do that.

 

That's just the thing. Imagine trying to fix that battery you would junk.

 

I wonder what the impact on our MPG is from a battery that will take 'as many amps' as the car can give it? After a HV battery rebuild, Dr Prius says my HV battery is (doubtfully?) better than new, but I have found that even after charging the Pb battery for 24 hours, the battery would only hold at about 12.3 v (after some hours of sitting). I have a desulfator hooked across plus and minus on the battery. The hope is that after some months the health is better. Unfortunately my MPG around town is only mid 40s, so I suspect about 5mpg going to the Pb battery in amps... I was thinking of switching in a smaller healthy 26AH AGM battery, while I run the desulfator on the tired 45AH Toyota battery on the workbench.

 

There isn't really any "fixing" of a 12V battery. If it is not holding a charge, just change it, especially if you have had a good run with it.

 

There is fixing: for batteries with covers, there is adding water, for lead plate, but also AGM, I believe, there is desulfating, plus there are some black arts techniques, which is why I noted that it is easier to repair the HV battery than to repair the aux Pb battery...