12V Battery - Charge Voltage

[Peter WA]

Since I've got my OVMS up and running I get an overview of the voltage of the 12V battery over time.

When the car is parked the voltage slowly drops, perhaps a little faster due to the OVMS itself and even faster since I've got a SIM installed and 3G is active.

The 12V battery appears to get charged in two different situations:

1) When the car is getting charged from AC. During that time the battery voltage is 13.2V.
2) When the car is in ready mode (i.e. driving). During that time the battery voltage is 14.2V.

For comparison: the alternator of my ICE car produces a reliably consistent 13.8V.

First of all: I am surprised to see those two different voltages. Are there two completely different circuits for the two scenarios? Or, if charging always goes through the DCDC converter, what could be the reason for that difference? (Being exactly 1V makes a typo in the DCDC software a possible explanation).

Second: neither of those voltages seems optimal, I'd be happy to see the 13.8V I'm used to from my ICE. At 13.2V the battery will never get fully charged, at 14.2V, the battery would get over-charged during long drives, reducing battery life expectancy.

Did anyone else notice the same or is this just my car?

 

[Coulomb]

The 12V battery appears to get charged in two different situations:

1) When the car is getting charged from AC. During that time the battery voltage is 13.2V.
2) When the car is in ready mode (i.e. driving). During that time the battery voltage is 14.2V.

That seems reasonable to me. That's a low float voltage for the possibly many hours of AC charging, where it will really only maintain its present charge, and a low charge voltage while driving. Does it stay around 14.2 V, or does it eventually drop into the 13's?

For comparison: the alternator of my ICE car produces a reliably consistent 13.8V.

It's something of a a simpler beast, I think, but I'll admit to not knowing modern alternators well.

First of all: I am surprised to see those two different voltages. Are there two completely different circuits for the two scenarios?

Both would be through the DC-DC.

Or, if charging always goes through the DCDC converter, what could be the reason for that difference?

As above, you don't want to be charging at too high a voltage for long periods of time, for fear of over-charging the auxiliary battery. I would hope that some computer is monitoring the current into the battery, so that when it reduces below a threshold, it is considered full, and the voltage would reduce to a float voltage (in the 13's).

Second: neither of those voltages seems optimal,

Ideally, the voltage should be temperature compensated: more voltage when cold, less when hot. Using the external temperature reading as the approximate auxiliary battery voltage seems a reasonable short-cut.
14.2 is reasonable, as it is just below the gassing voltage (14.4 V at room temperature). Above the gassing voltage, gases start to build up, and there isn't much volume to store them. Too much gassing will lead to loss of electrolyte fluid, which can't be replaced in a sealed battery.
13.8 is a bit low to charge the battery when it really needs it; it's really a high float voltage (suitable for colder temperatures).

I don't think the exact 1.0 V difference implies a typo in the control software source code.

Let's see if the voltage varies with temperature, and drops when the battery is deemed fully charged. EVs seem to get this fairly simple task badly wrong far too often. My other EV is a first generation Leaf, which is notorious for under-charging the auxiliary battery. It charges to about 14.4 V (probably temperature compensated) for a few minutes, then drops to a low float voltage, usually around 13.0 V. It does not seem to take into account the charge current, even through it's measured and is available on the CAN bus.

 

[Peter WA]

Thanks a lot for your very thorough reply.

I don't have much experience with charging car batteries I have to admit. I've got a couple of deep cycle AGM ones for camping and based on the documentation I got with them found 13.8V to be a nice safe 'in between' level without the need to always adjust the charger between times of heavy use and times of just maintaining them. When I found that my ICE alternator produces the same voltage I just assumed that they followed a similar logic.

Would make sense though if our cars, with so much logic used for HV charging, also spent a bit of time on optimising the 12V charging:

Does it stay around 14.2 V, or does it eventually drop into the 13's?

I'll check after the next longer trip and update here. Might be a while, no long trips planned soon.

 

[Paulie68]

Already covered above, so deleted!

 

[kenny]

13.8V seems to be the recommended highest charging voltage for an AGM.

i just joined up to see about this new EV that Mike is driving.

 

[Coulomb]

13.8V seems to be the recommended highest charging voltage for an AGM.

Howdy, Kenny! I wonder when the ZS EV will be available for y'all in the USA.

Re the AGM voltage, I don't know much about them, but I thought that they could take higher voltages than other sealed lead acid types. For example, the Optima in my Leaf can be charged to 14.7V with no current limit, and 15.6V when the current is regulated (under 51.7°C or 125°F). But it might be a special case.

I suspect that a lot of people use the term AGM when they really mean any sort of sealed lead acid battery. The inverter-chargers I work with have two built-in battery types: flooded and AGM. AGMs are limited to 14.1V, which seems too low to me for most actual AGMs. So I suspect that this setting is intended to be safe for all sealed LA batteries. I guess that 13.8V would be appropriate in very hot climates. There is also a user setting where the user provides their own bulk/absorb and float voltages.

13.8V is also a common maximum float voltage specification, but batteries tend to charge very slowly at float voltages.