Charging voltage for 12 volt battery

Your car is producing the "correct" voltage output. So, if you find that your driving patterns are such that the 12V battery is not kept fully charged, then you'll need to rely upon the external charger to top-off the 12V battery from time to time.

 

Yes, use an external charger (I suggest a Battery Minder at about $60) if your 12v battery doesn't get fully charged in normal use but use one that is limited to a 2 amp charging rate (our battery doesn't do well at higher charging rates).

JeffD

 

My 2004 NWH20 Prius charges at between 13.7 and 13.8V on the Scan Gauge (confirmed with a Fluke DMM) and a typical weeks use is 5-10km for 6 days and 150km one day a week going into Sydney. The battery goes flat enough for one of the computers to fail to boot up unless I charge it once a month (I forgot this month and it died outside the Post Office today). Interestingly it required more frequent recharges with one of the very expensive tubular electrode batteries than a medium cost maintenance free battery (which has gone back in) although I would have thought the chemistry would have been the same. As the duty cycle would seem to be closer to telecommunications use than typical heavy discharge automotive use I wonder if a telecomunications battery, which were traditionally float charged at 2.2V per cell might be more suitable.

Andrew Blakeley-Smith

 

Because the Prius battery is a sealed unit, with apparently no way to top up with distilled water, it's best to keep the charge voltage lower to avoid loss of water. The Prius will run at 13.8V normally. If you are seeing 13.9 it's likely the battery is getting old. It will still work for a year or two, but at least in Pearl, it shows the battery gets discharged easily, indicating less A-Hrs available. In my experience, this is because the electrolyte level is low.

As far as different chemistries, the different types of batteries do have slightly different float voltage values. I'm talking of sealed flooded lead-acid, flooded lead calcium, AGM (absorptive glass mat), and gel cell types here. Each is slightly different, and then there are other differences, such as recombinant technology, which will also slightly change the optimum float voltage. We're talking a few millivolts difference here, which may seem insignificant, but it can impact battery life. If you operate too low you won't shorten the battery life, you'll just get a little lower capacity in use. If you operate too high you will loose water. If you can't replace it you will shorten the battery life. Be aware that temperature (yearly average) will also affect optimum float voltage.

As far as charging the battery with an external charger, you WILL shorten the battery life charging at a higher voltage than the 13.8V the Prius system was designed for. This is because you will loose some water charging at higher voltage. If I had a choice, I would get a float charger that can be set to 13.8V.

As I have posted elsewhere, I have a "Battery Tender", designed for motorcycle and other small lead-acid batteries in over-winter storage. It floats at 14.2V, too high in my opinion.

 

That's strange Andrew. Even if you only drove the one 150km trip per week it should be plenty to keep the battery charged. You may have a stray load on your electrical system or a bad battery (though from your post it sounds like you've already tried more than one battery).

 

When I first had the problem I checked for any discharge with a clamp-on hall effect ammeter, suspecting a glove box light or something similar but nothing strange and the charging current seemed reasonable when the inverter started. I had thought of putting a silicon diode in the voltage sensing circuit so that the voltage control circuit thinks the battery voltage is 600mV less than it is and reacts accordingly but decided I could live with a quick charge every 3-4 weeks and now carry a 12V 7.2A/h battery in a carry case just in case. Apart from that it is an absolutely delightful little car, beautifully engineered and nicely offsets my 1963 Triumph Spitfire from the days of slide rule engineering

 

Check your resistance to ground, i.e. from the negative post to the chassis

 

I am guessing that your clamp on ammeter is looking at any field inductance around either the - or the + battery cables? Was the car totally off at the time of measurement? As stated you seem to have a load or parasitic current flowing ( When OFF ). You should not need to insert a diode. Does a clamp on measure DC? Seilerts suggestion is right on.

 

Yes, there are clamp-on current sensors that sense DC current. I have one and have used it on Pearl.

 

My wife's 2007 Prius failed to start this morning. Got the 12v battery boosted by AAA. Drove it to the Toyota dealer and it was confirmed that the battery was bad & they replaced it. However, I don't understand why my radio presets and auto window settings were not lost! Is there a way to prevent losing the settings when replacing the 12v battery??

 

Yes. All you have to do is maintain 12 volts on the bus while changing the battery. The easiest way to do that is to connect another 12V source to the jump points under the hood, and then swap out the battery. Once changed, remove the 12V source.

It's easy to do, but just be aware that the bus is still hot, otherwise you might carelessly short the positive battery lead to ground.

Tom