Prius charging voltage (READY mode) constant or varying? Indication of weak battery?

You don't have to guess by watching the charging voltage. Perform the built-in test procedure and know the battery's condition:
http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html

 

Possibly a force of ~14 volts always exist in the charging state and depending on the health and SOC of the 12V, the current flow varies from max to min, along with small pulses to maintain a certain level of charge. Each individual battery is going to have a internal resistance based on age. Should the charge rate be 14.1V AND the battery reads 14.1 volts ( after sitting ) this would be a perfectly ideal situation and probably never possible! Like us, from the moment of manufacture, a battery slowly deterioates.

 

Well, how much variation does this other individual notice? If the variation is substantial then I would say that it is more likely that his car has a problem, such as excessive resistance in the 12V bus which causes voltage fluctuations as current load changes.

I would say that your experience is normal. The DC/DC converter should maintain 12V bus voltage very near 13.8VDC at all times when the Prius is READY.

There is a "sense" wire connected to the 12V positive battery terminal which is supposed to tell the hybrid vehicle ECU to increase 12V bus voltage if the 12V battery is seriously depleted. However I haven't noticed this in effect, maybe because I do not allow the 12V battery in my cars to become depleted.

 

I've left my amateur radio on for a few hours at a time when the car was off and the 12V system voltage goes up to about 14V once I put it in "ready"(it almost always is at 13.8V otherwise). After a little while it goes back down to 13.8.
The radio draws just under an amp in standby.
There is no temperature compensation that I've been able to measure. The low battery condition seems to be the only thing that causes it to vary.

 

The 14,1 or the 13.8 Volts is only the force applied in an attempt to charge the 12 V battery, the charging rate has nothing to do with the real SOC of the 12 V Battery. That is determined after a rest, say overnight, and then read the voltage before putting it in ready. Sometimes putting a small load on first, like headlights a couple of seconds, will give you a more accurate reading. The best policy is to never run down the battery. If any accessory is used the car should be kept in ready mode ( engine on) :cheer2:

 

1) with add-ons like amps, inverters, radios, etc need deep cycle batteries.
The original battery is only 26 Ah and the added demand takes HP and puts more demand on alternator and charging system. A deep cycle battery is available from Optima or VPower (55 ah) at batteriesareus.com

2) alternators ARE NOT battery chargers, they are maintainers, and using them to charge batteries will reduce life of alternator and battery, and increase demand on entire charging system!!!

 

Ironically a higher charging voltage like that often means that the battery is in poor condition, with the higher than usual voltage being due to the weak battery's higher than usual internal resistance.

 

I think I am that person that you are referring to. I noticed when I bought the SG the 12 Volt voltage fluctuates while driving. I don't pay much attention to it these days and have had no problems. I did note the other day that it was at 13.something. I will pay attention the next week and note what it is doing these days.

On a related topic, correct me if I am wrong but the stock radio draws very little from the battery and would take days to run the battery down to the point that the car won't go into Ready mode. Not that I'd try that, but I don't believe listening to the radio for an hour with nothing else on makes much of a dent in the charge level of the battery. Wouldn't headlights draw magnitudes more than the radio?

 

Prius do not contain alternators in the normal automotive sense, so I'm not sure why this was brought into this discussion.

On a broader scale, alternators are used routinely to charge deep cycle batteries in marine applications without hurting the life of the batteries. Alternators are not the problem, it's the primitive voltage regulators used with many alternators that causes damage to batteries. Smart regulators are available that use the same charging profiles as shore-powered chargers.

Tom

 

Further, the Prius battery (I'm told) is actually a 38 AHr unit. It can be replaced with an Optima spiral wound AGM unit of about 40 AHr with an adapter kit from e-learnaid.

The Prius uses a battery charger built into the inverter. Holding the system voltage at 13.8 (or even 14.2) is not a "good" way to recharge a lead-acid battery, but it does work. It just takes some time. Expect to have to be in "ready" for at least 4-8 hours to get a good charge into the battery. The Prius battery is supposed to also be a "deep discharge" type. Some are said to be AGM units. Last I checked Pearl has unrestrained liquid electrolyte in her 12V battery (not an AGM type).

Trying to use another battery with jumper cables will cause problems. There is usually too much resistance in the connections/cable to result in a stable system. It will act strangely. Been there, done that.

The latest method for testing lead-acid batteries involves "pulse testing". It's a hand held unit that can cost from $200 to $2000. The trick with these is to ensure you get a good connection to the battery terminals with the meter test leads. The old load test method is still valid, but if you get someone to do it keep the max. current load to 100 A (connected directly to the battery, of course).
Either of these methods are the best way to determine if your battery is limping badly. A final test would be to see if the battery looses a lot of charge overnight. If either (or both) of these tests indicate a failed battery replace it.

 

The battery voltage is one of the 4 gauges on my ScanGuage so I look at it occasionally as I drive. I have noticed this voltage fluctuation as well and have a newly installed Optima. The orginal OEM battery also showed fluctuations when driving so it isn't something new, for me at least. The Optima fluctuation is about the same as the OEM was.

Sometimes while in ready it will bounce from 13.5 to 13.7 and stuff then settle on 13.7/13.8. Other times it will bounce just on 13.7/13.8 others still 13.4/13.5. Very rarely will it go below 13.4 or higher than 14.1 while driving with the following exception.

If I start the car with nothing on, no lights no A/C, nothing and the only thing showing on the dash is "Ready" the voltage will sit at 13.1/13.2 for as long as I watch. If I turn the A/C or lights on the voltage instantly jumps to 13.6 - 13.8, range. I always find that interesting.

I have never had the battery dead, never had to be jumped, never had any of the weird symptoms so I think it might be as much an issue with the ScanGauge as with the battery. I have turned it off, the ScanGauge, while driving down the road and before the "reboot" it read 13.5 Volts and after 13.8 Volts. Nothing should have changed in the car so why is the voltage different?

You could go nuts thinking about it so I use the ScanGauge as intended, an occasionly monitor to verify various states of health NOT as an instantaneous monitor from second to second. At least not for the battery gauge, LOL.

Happy driving,
Chris

 

My Scangauge also has the 12V system voltage as one of the gauges. I don't see the variation you report. Perhaps you have a resistive connection somewhere in the circuit? Battery posts were cleaned before you connected the Optima?
What you describe is what you would see if there were oxide under one of the connections. A high current draw would spark over it and until it was allowed to rebuild the oxide (overnight) it would run as if properly connected.

 

I've had a scangauge in my 2008 since it was new. From day one, 12V battery voltage would sit at 13.7 to 13.9 volts as long as there was any draw on the system, (AC, lights. etc.) If none of those things were on and the battery were fully charged it will drop to 13.2 volts. Seems like when the DC-DC converter current draw drops below a certain level it drops it's output to 13.2. Probably better for the battery once it's fully charged.