Harbor Freight Battery Maintainer Mod

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

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.

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.

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

Someone here had a HF maintainer smoke a battery,....but mite have been more to the story. Both my HF maintainers sit at 13.3 never more than plus/minus 1/10

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 :)

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 - 13.4

Even 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

Nice mod!!!

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.

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.