I have a TT that I store at a remote location during the winter and I bring the group 31 AGM battery home with me. I have a charger to ensure that it is at 100% SOC before it gets set in the corner of the garage, but I have been looking for a float charger to keep it at the mfr's recommended float voltage of 13.4-13.6V.I found a coupon from Harbor Freight for one of their 69955 Automatic Battery Float Chargers for only $5 and figured it was worth a shot to see how well these little maintainers perform. However when I hooked it up it was apparent that there were two significant flaws; 1) The LED power indicator would illuminate even when mains power was disconnected and 2) the float voltage was in the 13.8-13.9 range.After cracking the case open I realized that both of these issues could be easily remedied with components I had in sitting on on my workbench…. I added a generic 1A Schottky diode to the output wiring which prevents the battery from powering the LED in the event of a loss of AC power. I replaced resistor "VR1" with a 100 Ohm 22-turn trim-pot which gives me control over the charge voltage.I'll keep an eye on the voltage & current over the next few weeks, but at the moment this looks like a promising battery maintenance solution.Cheers-MarkDiode & potentiometer installed.The trim-pot can be accessed from outside the enclosure.AGM Battery float-charging at 2.41mA and 13.47V.

Nice mod.i've been thinking a 12v wallwart transformer could be purposed for this use. I see the circuit in the HF model you used is really simple. I may have to investigate this further, I have more than a few wallwarts laying about.

An AGM battery does not need a maintainer if fully charged when put in storage. The self discharge rate is only 1-2% per month.Maintaining an AGM battery at 13.8-13.9 you will be risking thermal runaway, that will destroy the battery.

There are a bunch of 800ma 15 volt wall warts out there. One 3-amp silicon diode on the positive, another on the negative will drop voltage around 1.5 volts. Rendering a 13.5 final but the 800ma unit will sag to around 13.3 - 13.4Even unregulated warts can be used if they are 800ma. Silicon or shottky diodes can used to fine tune finishing voltage. Verify voltage after 24 hours.VRB construction only has problems with thermal runaway if a wattage threshold is breached in combination with excessive internal temperature. I'm afraid 12 watts is far too menial an energy source to provoke catastrophe. By maintaining a finishing float voltage in the 13.2 to 13.5 range

westend wrote:Nice mod.i've been thinking a 12v wallwart transformer could be purposed for this use. I see the circuit in the HF model you used is really simple. I may have to investigate this further, I have more than a few wallwarts laying about.I use a 1A 12v wallwart modded with aligator clips. Works great :)

Harbor Freight Battery Maintainer Mod | Good Sam Community

MEXICOWANDERER
Explorer
Oct 02, 2014
Post Script

800 ma HEAVY transformer type 12 volt wall warts are a dime a dozen. I would not pay more than a dollar each for a used one. There are those retailers out there that self proclaim themselves as the "Hunt Brothers of Wall Warts" and ask stupid prices. Remind them the heavy transformer type 800 milliamp wall warts are UNREGULATED. If they still won't deal, walk away. Salvation Army and other thrift stores are great sources for cheap wall warts. No one wants the heavy plug in type.

These 800 ma transformer wall warts SELF BALANCE (meaning limit) voltage when coupled to a car battery. Double check voltage after a few days just to be sure.
MEXICOWANDERER
Explorer
Oct 02, 2014
when a one amp or greater potential Wall Wart is putting out 15 volts a silicon diode is warranted. When the wall wart is rated for 13.5 to 14 volts a SHOTTKY is mandatory.

A rectified 800 ma transformer wall wart is ideal. Open circuit voltage may be as high as 18.0 but operating through the impedance of a single battery finishing voltage will be around 13.45 A group 24 slightly higher a group 29, 30, or 31 slightly lower. An AGM battery has not be tested yet.....forthcoming.
opnspaces
Navigator II
Oct 02, 2014
Thanks, I'll pick up a few of the cheap Radio Shack diodes. I'm usually pretty thrifty but I guess I can spare $1.50 each for an experiment.

My guess though is that as you say it will drop the charge voltage too low. But hey it's something fun to try and the parts are cheap, so why not? :)
MEXICOWANDERER
Explorer
Oct 01, 2014
Don't sweat the shottky voltage. A five amp 40 volt is overkill to the max. Shottky's do not like abuse like lots of ripple and noise. A 100uf 50 vdc electrolytic capacitor connected across the positive and negative output charging wires will make output voltage as smooooooooooth as a baby's butt.
msiminoff
Explorer II
Oct 01, 2014
opnspaces wrote:
Could someone on here give me a lead to a description and maybe a part number that I can pick up a diode? Or can I just go to my local Radio Shack and pick up any 1A diode from their parts drawers?

You don't need to use a Shottky diode… I just happen to have a bunch of NTE578's and this type of diode is suitable for the job.

However, if you have a RadioShack store nearby I suggest that just get a plain ol' (read: cheap) 1N4001 diode.

Keep in mind that if you choose to add the diode then you may also need change the value of resistor VR1 (alternatively you can add the 100 Ohm trim-pot as I have done), otherwise the voltage drop (~0.7V) across the diode may cause the charge voltage to be too low (depending upon your desired float voltage of course).

Cheers
-Mark
KJINTF
Explorer
Oct 01, 2014
Got several of those at HF - believe I paid $2.99 each when they were on sale
To keep it simple I use a diode in series with the output - gives me a constant 13.2V. For the price they can not be beat!
Monaco_Montclai
Explorer
Oct 01, 2014
wal-world has one on sale for 24usd I getting it, now its all happy -camping
westend
Explorer
Oct 01, 2014
These schottky diodes are available in four different packages and even though the voltage rating is 150V, they are suitable for switching power supplies with "rail voltages to 24V", according to the data sheet.
opnspaces
Navigator II
Oct 01, 2014
I've got around 5-6 of those maintainers floating around my garage and I've also been bothered by the LED being on when power was disconnected. I figured it could use a diode, but not being sure which diode to get I just left it alone. After this thread though I figure I can do it too.

I looked online for 1A Schotky diodes and I see them in 30 and 40V ranges which seems like it would be too high. Could someone on here give me a lead to a description and maybe a part number that I can pick up a diode? Or can I just go to my local Radio Shack and pick up any 1A diode from their parts drawers?

Thanks.
landyacht318
Explorer
Sep 29, 2014
Thanks for the write up and Pics.

I've red elsewhere that the voltages on these HF units vary wildly. Luck of the draw, or better have the skill to modify.