# constant float charging a lead acid battery.



## degarb (Nov 2, 2011)

I was under impression a car that sits should have a float charger on it. However, I seem to have killed two or 3 batteries this way in last year.

Could it be brand of float charger (walmart)? Or should I just float charge for 1 day every week?


----------



## JacobJones (Nov 2, 2011)

Probably just a crappy float charger with no intelligence, it'll sit there quite happily pumping power into a fully charged battery boiling away the electrolyte, find an automatic one that stops when the battery is full and then restarts when the voltage drops.


----------



## Russel (Nov 2, 2011)

The key with float charging a lead acid battery is the voltage. 13.8V on a standard car battery should be ok, but any more than that may cause problems. Lead acid batteries start gassing at 14.4 volts, so that is definantly to high. I used a small lead acid battery as a UPS for a car stereo system I set up in a shop. I had it rigged so that the regulated power supply (13.8V) passed through a diode before reaching the battery and the care stereo was powered directly from the battery. Due to the voltage drop from the diode, wiring and connectors, the power supply voltage reaching the lead acid battery was 13.5 volts. The battery lasted for years without any problems. The shop got lots of short power outages, so with this system we always had our music, and didn't lose the programed radio stations in the car sterio.


----------



## Norm (Nov 2, 2011)

Check out the range of Ctek Battery Chargers, there are other brands, you need a charger especially designed for maintenance charging.
Norm


----------



## xul (Nov 2, 2011)

If you run a car 2 hrs/day it probably gets float charged 2 hrs/day. Restoring the charge lost from cranking takes very little time with a 60A alternator.

What's the self discharge rate for a car battery in your climate? Add to this 0.01A lost continuously. Once/week restore this lost charge. 0.01A for 168 hrs is 1.7 A-h of charge.

But watch out for this logical fallacy
http://en.wikipedia.org/wiki/Post_hoc_ergo_propter_hoc


----------



## Russel (Nov 2, 2011)

xul said:


> If you run a car 2 hrs/day it probably gets float charged 2 hrs/day. Restoring the charge lost from cranking takes very little time with a 60A alternator.[...]



As I understand it, float charging is applying a fixed voltage to a battery and maintaining that voltage. If the voltage is set right for the lead acid battery, the battery can remain on the float charge 24/7. A cars charging system (when the car is running) is different. It will apply a much higher voltage that what would be approriate for float charging.

The advantage of float charging at say 13.5 volts is that the charge current will drop to almost nothing when the battery is fully charged. The battery can be maintained at 13.5 volt indefinately. As the battery self discharges, the voltage of the battery will drop slightly and it will take some current from the regulated float charger. The problem, I suspect, is that some cheaper float chargers use a voltage that is too high. 

I was surprised, for example, when I checked the maintenance voltage on several small APC uninterruptible power supplies that I have. When the UPS is on, meaning that there is 115V output from the device, the "float" voltage to the internal battery was about 13.6 volts. But, when the main switch was turned off, meaning the 115V output is off, the battery float voltage went up to almost 14 volts. That is too high, so I alway keep my little UPS supplies switched on so that they arn't overcharging the internal sealed lead acid batteries.


----------



## xul (Nov 2, 2011)

Here's another bookmark I'll have to read
http://en.wikipedia.org/wiki/Trickle_charging

If a fully charged battery is 12.76v then anything above this will put current into the battery, limited by the battery's internal resistance while charging.
For cars in the summer, 13.3v will work and in the winter up to 15.5v is allowed. Usually I read 14.4v or so.

When I had a car with an ammeter the needle was always on the slight positive side during normal use.

With my Subaru they cut things pretty close; sometimes the engine cranks very slowly.

Bosch put out a book on automotive electrical systems and they had a strange looking chart on battery sizing vs. alternator size vs. driving habits. 
It would be easier if carmakers weren't trying to shave every penny off the production cost of their cars.


----------



## Russel (Nov 2, 2011)

Interesting...

Float voltage - Wikipedia

According to the information at this link, the correct float voltage for flooded lead acid batteries is 13.4 volts. Well, I guess I was a little high at 13.5 volts, but not by much. Also, as Xul pointed out, battery temperature is an important factor. I live where the climate is very moderate, so temperature isn't a big factor here, but if you live where it gets very cold or very warm you might need to account for it when "float" charging a lead acid battery.


----------



## Lynx_Arc (Nov 2, 2011)

One thing you could do with a float charger is use a 24 hour timer and limit the "on" time to something more appropriate.


----------



## MikeAusC (Nov 3, 2011)

Russel said:


> Interesting... Float voltage - Wikipedia According to the information at this link, the correct float voltage for flooded lead acid batteries is 13.4 volts. . . . .



Oversimplification like that will kill a battery in the real world.

A Float voltage that works fine for Calcium-Calcium battery will kill a Lead Antimony battery.

A Float voltage that struggles to charge to a battery on a winters night will cook the battery on a summers day.


----------



## Russel (Nov 3, 2011)

MikeAusC said:


> Oversimplification like that will kill a battery in the real world.
> 
> A Float voltage that works fine for Calcium-Calcium battery will kill a Lead Antimony battery.
> 
> A Float voltage that struggles to charge to a battery on a winters night will cook the battery on a summers day.



I've had the same flooded lead acid battery (non maintenance free) float charging on a regulated 13.8 volt power supply (with a diode dropping the voltage to the battery to 13.5 volts) for years without any problems. As I said before, the climate here is very moderate. If the lead acid batteries see temps that vary more than from 50 to 75 degree fahrenheit, you will need to account for that in order to maintain your battery.


----------



## xul (Nov 4, 2011)

Also, underhood ambient temps for a running engine are higher than outside ambient in all weather.


----------



## samgab (Nov 6, 2011)

Norm said:


> Check out the range of Ctek Battery Chargers, there are other brands, you need a charger especially designed for maintenance charging.
> Norm



I agree with Norm. Prior to buying a car charger, I did a huge amount of research and reading, and got books out of the library etc.
I ended up getting a CTEK MXS 5.0.
It's a very smart charger.
*For initial charging, you can choose 5Amps or 0.8Amps - whichever is more appropriate due to the capacity of your battery.
*It does a desulphation pulsing phase if necessary.
*It has a soft start mode if necessary, if the battery was overdischarged at first.
*It follows correct CV/CV method. runs at full current until voltage reaches 14.4V, then the current decreases to about 100mA whilst maintaining the charge V at 14.4.
*Once charged, it analyzes if the battery is good/maintains charge.
*If so, it drops to a float charge of 13.6V.
*Even a 13.6V float charge shouldn't be kept on forever, so for really long term storage, if the charger remains connected, after about 10 days it goes to long term pulse mode, where it monitors the battery voltage, and if it gets down to 12.7V, it will do a pulse charge to keep the battery between 95-100% charge.

It follows the "best practise" that I'd read about in pretty much all the books I read on the subject.

There is also a Recondition mode, that should only be used on batteries in poor shape, or once a year at most. It applies a carefully controlled overcharge at 15.8V for a short period of time, causing the acid to "boil" which mixes the electrolyte thoroughly and can restore a battery in some cases. I don't use this mode, because my battery doesn't need it.

For those in the USA, I think the MUS 7002 does pretty much the same thing.


----------



## Kilted (Nov 6, 2011)

Also check out these guys; http://www.batteryminders.com/batterycharger/catalog/All-orderby0-p-1-c-255.html they have battery float chargers that are solar powered plus the usual 120VAC.

=D~~ Kilted


----------



## Exit32 (Nov 7, 2011)

I've had good luck with battery maintainers bought from Harbor Freight for little money -- often $4.99 when they're on sale. I use them on several classic cars stored in a heated barn during the winter, and the batteries have always been ready to go when Spring arrives.

I've got another cheap Harbor Freight maintainer connected full-time to a cheap Walmart lead-acid battery that's hooked to a 10,000-watt whole-house emergency generator. The battery is probably seven or eight years old now, and it always starts the generator without complaining.


----------



## AIRASSAULT18B (Nov 7, 2011)

I run banks of 6v golf cart batteries for my RV. I have ATV & MC batteries, AGM & Gel Cell hooked up to multiple Battery Tender stations. I have used them for over ten years & have not had to replace a battery or add water since they have been hooked up. They may cost more than some of the ones mentioned but, when you have a couple of thousand in vehicle batteries you do not want to take any chances. Look here http://batterytender.com/ find what you want & try Amazon etc for a cheaper price.


----------



## degarb (Nov 10, 2011)

I stumbled on http://www.micro-power.com/ironworks Looks like just short of auto application, not sure. Wonder where these would fit in around agm, pricewise.


----------

