Tired of replacing house batteries...

[Geoff]

What is the best float voltage for a 12v house battery bank?  My Trace SW2512MC thinks it should be 13.8 volts but I find that 13.8 is too high and boils out my golf cart battery bank.  I am tired of replacing batteries every two-three years.  I have reset the float voltage to 13.2 instead of the default 13.8 and am hoping for the best.

[bioVenture]

How many batteries do you have and how are they configured?

[bioVenture]

I haven't worked with these three stage units, but what I do know is that in automotive applications a typical float voltage from an alternator on a 12v system is usually around 12.9 -13.2 so your on the right track by lowering it down.

[David Anderson]

Hey Geoff, haven't heard from you in a long time.  Glad to see you back. 

I had a similar problem.  I boiled my Sam's golf cart batteries dry twice.  I reset to 13.2 float on the SW2512 and it stopped.  I have an 8 battery array.  They lasted 6 years before I replaced them.  They were definitely a bit fatter after six years of sulfation.  I would equalize at 15.6 volts every couple of months.  I just loosen the caps and throw an old towel over the top to catch the splatter so it doesn't get on anything in the bay.  Give those fumes lots of fresh air. 

David

[Sean]

What is the best float voltage for a 12v house battery bank?

The answer to this question is always: whatever the battery manufacturer specifies.

All battery manufacturers publish the recommended charger settings for their batteries.  Even among similar batteries of similar construction, different manufacturers may specify different settings (and a single manufacturer may specify different settings for different models).  Your retailer should have provided you with a copy of these at purchase time, but, if not, most manufacturers also publish this information on their web sites.  Note that there are hundreds of brands of batteries but only a handful of actual manufacturers; if you can't find the specs on the site for the brand name, try the manufacturer.  Bill Darden's excellent web site has a cross-reference:
http://batteryfaq.org/

My Trace SW2512MC thinks it should be 13.8 volts

I would guess somebody told it that somewhere along the line, because the default float setting on an SW2512 for Lead Acid battery type is 13.4v.  That said, you should never accept the default settings unless there is no choice; always use the settings recommended by the battery manufacturer, not the charger manufacturer.  That goes for bulk voltage and absorption time as well as float voltage.  The Trace also has a configurable equalize voltage and time, and recommendations for these parameters can also be found with the battery manufacturer.

but I find that 13.8 is too high and boils out my golf cart battery bank.  I am tired of replacing batteries every two-three years.  I have reset the float voltage to 13.2 instead of the default 13.8 and am hoping for the best.

Absent any information about your specific batteries, the lead acid "standard" setting for float is 2.20 volts per cell, or 13.2 volts.  So you should be safe with this setting.

All that said, note that flooded batteries require religious attention to maintenance.  They must be equalized monthly, checked and watered weekly at minimum and preferably daily, and true deep cycle batteries should not be left on a float charger indefinitely; cycle them to at least 20%-30% DoD at least monthly.

If you have flooded batteries I also recommend checking the electrolyte specific gravity periodically; catching one cell starting to go bad can prevent catastrophic damage to the whole bank by retiring that battery from service.

How many batteries do you have and how are they configured?

The number and configuration of batteries has no impact on the float setting.  In fact, on the Trace SW series, it has no impact on any of the charge voltage settings -- it only affects the "maximum charge current" setting which applies to the bulk cycle.  Max charge current should be set to 0.1*(C₂₀/5) for flooded batteries, and 0.1*(C₂₀/2) for AGMs (on a 12-volt system; use 0.2 instead of 0.1 as a multiplier for 24-volt).

In plain English, that means set your DC bulk charge rate, in amps, to one-fifth the amp-hour capacity of the bank for flooded, and one half for AGM.  The SW series, however, sets its bulk charge rate in AC amps at 120v, so you need to divide the DC rate by ten for 12v, or 5 for 24v.

As an example, if your battery bank is 450 amp-hours of flooded batteries at 12v, then the maximum DC charge rate would be 90 amps.  To limit the charger to this rate in the bulk cycle set "Max Charge Amps AC" on menu 10 (Battery Charging) to 9 amps.  If you left the setting at the factory default of 20 amps, you'd boil the batteries dry in short order.

... in automotive applications a typical float voltage from an alternator on a 12v system is usually around 12.9 -13.2 ...

Actually, automotive alternators are not, technically, "float" chargers.  They are a compromise between bulk and absorption settings.  More importantly, they are set and designed to charge starting batteries, not deep cycle batteries, and the charging requirements are very different.

BTW, I disagree with your assessment of what a "typical" alternator setting is; most automotive alternators are set near 14 volts (or 28 volts on a 24 volt system).

Alternators on buses are generally externally regulated with an adjustable regulator;  if you are using your alternator to charge the house bank you will probably want a setting of at least 13.5 (27.0) and probably closer to 14.0 (28.0) in order to get enough charge into the bank while under way.  Three-stage charging regulators are available as well.

-Sean
http://OurOdyssey.BlogSpot.com

[bioVenture]

yes that is true about alternators not floating, alternator do not do this because the are never in a no load condition.  The misconception of alternator being set around 14v is due to the load the alternator handles to recharge and power accessories.  (in light duty automotive) A fully charged battery, above 70 deg will level off the regulators duty cycle to around 10% when no accessories are in use will leave the voltage lower than 14.  otherwise the battery will boil.  These systems are a different animal than light auto though 

[rv_safetyman]

OK, this subject came up a while  back and I had just lost a battery (think because I did not keep up with maintenance and it got pretty dry). 

I recall the discussion about float voltage and recall playing with the setting.  However, I recall that temperature was a pretty significant factor.  I recall checking the TriMetric several times at different ambient temperatures and the float voltage changed pretty significantly as I recall.

My pea brain has lost most of the details, and perhaps confused the whole story.

Any comments on ambient temperature impact on float voltage?

Interesting comments on doing some cycling (I leave my batteries on the inverter float between use) and also on equalizing.  I have tried to find a time specification for equalizing (Exide golf cart).  I could never find a good answer.  For that matter, the voltage level also seems to be a mystery to me.

Jim

[Chaz]

So then should the float then be 26.4 on a 24v system?? I'll have to check what my 4024 Zantrex is set at.

This is a good thread as I can't afford to be replacing my ( 6v house batteries very often. But are you serious about how much maintenance is necessary?? MAN!! I guess I'm a slacker.

Chaz

[Sean]

.. I recall that temperature was a pretty significant factor.  ...
Any comments on ambient temperature impact on float voltage?

Yes, temperature (of the batteries themselves, which is affected by ambient, of course) is important.  Warm batteries work better than cold ones, at least up to the point where the heat can damage them.

Really good chargers, including the one in the Xantrex/Trace SW series inverters, have temperature-compensation built in.  In order for it to work, though, you need to purchase the temperature sensor and attach it to the batteries.

Good SOC meters also have this feature with the (usually optional) temperature sensor.

I believe there is a discussion of temperature effects on Bill's site, linked above.

So then should the float then be 26.4 on a 24v system??

Again, you should check with your battery provider.  But if you have traditional flooded plain lead-acid batteries, 26.4 is probably the correct float setting.  If you go to Bill Darden's site that I linked above, he has charge profiles for common battery types laid out graphically, in volts-per-cell.

Note that AGMs use different settings (although they will generally not be damaged by using flooded settings).  Gells have vastly different settings and can be destroyed on flooded settings.

-Sean
http://OurOdyssey.BlogSpot.com