Harbor Freight Battery Maintainer Mod

[msiminoff]

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
-Mark


Diode & potentiometer installed.


The trim-pot can be accessed from outside the enclosure.


AGM Battery float-charging at 2.41mA and 13.47V.

[MEXICOWANDERER]

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]

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]

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]

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]

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]

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]

wal-world has one on sale for 24usd I getting it, now its all happy -camping

[westend]

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]

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]

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.

[MEXICOWANDERER]

I've been fooling with these gizmos for years. The best thing to look for in a junk store is a 800ma HEAVY cube shape wall plug-in rated 15 volts. Extremely common, because it is a transformer unit. The -average- level of self-regulation load versus potential is 13.45 volts when a 3-amp silicon diode is used in line.

Switch type wall warts are different. An individual needs to double-check voltage potentials for float.

A 58 cent wall wart is going to babysit my new lifeline. I figure the AGM's lower impedance will lower stasis voltage to around 13.10 - 13.20

[mena661]

I have a couple of these maybe I'll try the same thing just for the hell of it. Great info and thanks for the post.

[westend]

Hey Mark,
Good stuff and the financials are evident, I didn't know they gave the things away. Heck, a good set of clips approach $5.
I'm looking around in the bins to see if I have a bourns pot that will do the job.

Up here, in the Great White North, battery maintainers are like gold. I can think of three batteries I have that could use maintainers.

[msiminoff]

Yes I do agree (westend, Mex', & phunk)… a similar result can be achieved with a wall-wart. But keep in mind that my total investment is about $6. There's no way I could assemble all of the components myself for anywhere near that price. A new regulated or switching wall power supply alone would cost me $10-15, and still wouldn't give me the ability to adjust the charge voltage or limit the current to my personal preference (ie. battery mfr's spec').

This super-cheap HF maintainer incorporates a 78XX linear voltage regulator and a TIP41 to regulate current. In addition it has a S9013 transistor to turn itself off if the battery voltage is too low.
As I mentioned, in stock form it does have some minor drawbacks, specifically;
1) A fixed output voltage, which may or may not meet my needs (I have read reports from other users that indicate voltages as low as 13.1 and as high as 14.1. Mine was 13.8-13.9)
2) There is no protection to prevent the device from discharging the battery if mains power was lost (or more likely if/when the wall transformer fails).

These 2 issues are easily fixed with the addition of one potentiometer and one diode… about $1 worth of parts.

The fact remains that this maintainer does not address temperature compensation at all :M Conveniently I live in the SF Bay Area where we have very modest seasonal temperature change. My workshop is climate controlled and the voltage I selected (~13.45V) will be fine for up to around 80ºF.

harold1946 wrote:
An AGM battery does not need a maintainer if fully charged when put in storage.

Ha, I guess I should said "in storage for 11 months".
At any rate, a well-regulated (bonus points for temp compensated!) float charge is always preferable over leaving a battery to self-discharge while in storage.

Cheers,
-Mark