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HOW TO: DIY Battery Tender for under $10

13K views 31 replies 15 participants last post by  SparkyF250 
#1 · (Edited)
If you're like me and don't like paying $$ for something that can be done cheaper, read below. The de-facto standard is the Battery Tender. However, it is upwards of $50.

Enter the DIY solution:

The optimal trickle charge for a vehicle at 12v is between 1amp and 1.5amps. What do we have here? A charger that outputs 1.3amps and has overcharge protection? Perfect - Car Truck Motorcycle 12V Smart Compact Battery Charger Tender Maintainer NEW
(note, I tested on my craftsman digital multi-meter and it puts out 1.27amps and does stop charging when the battery is full.)

EDIT: For those that don't know, overcharge protection means that the charger is smart enough to know when the battery is full, and switch to just a maintenance mode. If the battery drops below full, it swaps back to "charge mode"

But wait, alligator style clips don't help us. Right you are. What we need is a way to permanently attach leads to the battery with a quick disconnect.

$2.18 with free shipping AND its fused for us? FANTASTIC. Battery Tender SAE DC Power Automotive DIY Connector Cable with Fuse 68cm

Finally, we need a way to get the power from our charger to the quick disconnect plug. That'll run you a whopping $1.18. Battery Tender SAE DC Power Automotive DIY Connector Cable 2x0.75mm 30cm 300mm | eBay


1. Take charger, cut off alligator clips
2. Strip back approximately 1" of wire
3. Red to Red, Black to Black with the bare end quick disconnect wires. Solder or twist together. Cover with heat shrink or electrical tape
4. Install the ring terminal connections to the battery (the nuts are 10mm for removal). Tuck the extra wire and the fuse into the battery box. Run the quick disconnect out from the battery box and allow to hang out with the grounding cable that runs through the grommet
5. Plug your charger into the quick disconnect and into the wall.

Total cost (assuming you have some heat shrink or electrical tape laying around) - $7.xx

When you are ready to go for a drive, just unplug, close the hood and take off.


I've got 4 of these that I made and they've been running perfectly for about a year now.
 
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#2 ·
Nice!!! How about some pictures?

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#4 ·
Ask and ye shall receive.

I like the DIY and thanks for the write up

I hate to be negative but when i see cheap china electronics with I get nervous
the last item i bought smelled like burnt wire insulation as soon as i cut the shrink wrap

and sure enough the thing didn't work

if you buy a tender like this it is ready top go they can be had for less than 27 if you shop smart
There's always a Chinese product naysayer. From the link I sent, I have 4 of them running constantly. They're all over 1 year with no issue. For $27, I can build 4 of them instead of 1 battery tender. Guaranteed that most of the electronics in the battery tender are coming from China or Taiwan. You're paying the $$ for the name and the warranty. For the difference in price, I will self-warranty.
 

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#3 ·
I like the DIY and thanks for the write up

I hate to be negative but when i see cheap china electronics with I get nervous


the last item i bought smelled like burnt wire insulation as soon as i cut the shrink wrap

and sure enough the thing didn't work

if you buy a tender like this it is ready top go they can be had for less than 27 if you shop smart

https://www.amazon.com/Battery-Tender-021-0123-Junior-Charger/dp/B000CITK8S

 
#12 ·
The man has 4 that have worked for a year so why be worried?

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#15 ·
most of the name brand chargers are made in china too
for a couple of bucks more name brand charger has a "NAME " and USA address
aka accountable for fire plus a warranty

good luck proving the DIY charger you modified was accountable for a fire or battery damage

half the china stuff i see as components does not test to the specs they list so at least dbl check

Contact Us | Deltran Battery Tender

I also buy in to this

Battery Tender® Jr.
1. How is the Battery Tender® Jr. battery charger different from a trickle charger?

The Battery Tender® Junior (BTJR) battery charger has microprocessor controlled power electronic circuitry which enables it to preform and safely control a number of sophisticated charging functions, well beyond the capability of inexpensive trickle chargers. Some legacy marketing literature refers to the BTJR as a “Trickle Charger with a Brain”. That description was based in context on two parts, first the relatively low output current, and second, the reduced level of charge control sophistication on earlier BTJR models relative to the BT Plus. Since mid-2006 the only major functional difference between the BTJR and the BT Plus is the maximum amplitude of the charger current, 0.75 and 1.25 amperes, respectively. Even though the marketing description may still be applicable, again, in a limited context, we can say now that the BTJR has a larger brain that enables it to create maximum charge effectiveness with minimal output current amplitude.

After connecting the BTJR to a battery and then applying AC power, it first conducts a number of checks during Initialization Mode to ensure that the battery functioning normally. Then it will deliver its full charge at a constant rate of 0.75 amperes. This is called the Bulk Charge Mode. The battery voltage will rise and when it reaches a predetermined level the BTJR will hold the battery charge voltage constant at that level, allowing the charge current amplitude to drop. This is the Absorption Charge Mode. The Absorption Charge Mode is complete when the battery charge current drops below a very low value, usually below 1/8 ampere. Some BTJR models have timers to limit the duration of the Absorption Charge Mode.

After the current drops or the allotted time expires (typically several hours), the BTJR automatically switches to a Float / Maintenance Charge Mode. The purpose of the Float / Maintenance Charge Mode is to maintain the battery voltage just slightly (typically between 1/10 and ½ volt) above where it would be if it were fully charged and sitting at rest. This keeps the battery topped off at voltages well below the gassing voltage of a lead acid battery.

Based on price alone, trickle chargers often appear to be a better economic choice for the typical consumer, but trickle chargers do not have the advantage of sophisticated electronic control. Therefore, as they allow the value of charge current to trickle down to what appears to be safe levels, the output voltage of the charger may very well rise to an unacceptably high level, sometimes even going higher than 16 VDC depending on the charger type and the battery that is connected to it. This magnitude of voltage is far above the gassing voltage of a lead acid battery. If the battery remains connected to this high level of voltage for an extended period of time, extreme damage may be done to the battery. Without Battery Tender® type electronic safety controls, what appears to be an initial cost savings for the charger may actually cost several times the charger price in replacement batteries
 
#16 ·
for a couple of bucks more name brand charger has a "NAME " and USA address
aka accountable for fire plus a warranty

good luck proving the DIY charger you modified was accountable for a fire or battery damage
...good luck getting battery tender to take responsibility for a fire. They'll make you prove that their product was to blame, which will be basically impossible.



half the china stuff i see as components does not test to the specs they list so at least dbl check

Go back and re-read my first post. I did test. They all came in at spec. Which is why I linked to the exact one I ordered.



Contact Us | Deltran Battery Tender

I also buy in to this

Battery Tender® Jr.
1. How is the Battery Tender® Jr. battery charger different from a trickle charger?

The Battery Tender® Junior (BTJR) battery charger has microprocessor controlled power electronic circuitry which enables it to preform and safely control a number of sophisticated charging functions, well beyond the capability of inexpensive trickle chargers. Some legacy marketing literature refers to the BTJR as a “Trickle Charger with a Brain”. That description was based in context on two parts, first the relatively low output current, and second, the reduced level of charge control sophistication on earlier BTJR models relative to the BT Plus. Since mid-2006 the only major functional difference between the BTJR and the BT Plus is the maximum amplitude of the charger current, 0.75 and 1.25 amperes, respectively. Even though the marketing description may still be applicable, again, in a limited context, we can say now that the BTJR has a larger brain that enables it to create maximum charge effectiveness with minimal output current amplitude.

After connecting the BTJR to a battery and then applying AC power, it first conducts a number of checks during Initialization Mode to ensure that the battery functioning normally. Then it will deliver its full charge at a constant rate of 0.75 amperes. This is called the Bulk Charge Mode. The battery voltage will rise and when it reaches a predetermined level the BTJR will hold the battery charge voltage constant at that level, allowing the charge current amplitude to drop. This is the Absorption Charge Mode. The Absorption Charge Mode is complete when the battery charge current drops below a very low value, usually below 1/8 ampere. Some BTJR models have timers to limit the duration of the Absorption Charge Mode.

After the current drops or the allotted time expires (typically several hours), the BTJR automatically switches to a Float / Maintenance Charge Mode. The purpose of the Float / Maintenance Charge Mode is to maintain the battery voltage just slightly (typically between 1/10 and ½ volt) above where it would be if it were fully charged and sitting at rest. This keeps the battery topped off at voltages well below the gassing voltage of a lead acid battery.

Based on price alone, trickle chargers often appear to be a better economic choice for the typical consumer, but trickle chargers do not have the advantage of sophisticated electronic control. Therefore, as they allow the value of charge current to trickle down to what appears to be safe levels, the output voltage of the charger may very well rise to an unacceptably high level, sometimes even going higher than 16 VDC depending on the charger type and the battery that is connected to it. This magnitude of voltage is far above the gassing voltage of a lead acid battery. If the battery remains connected to this high level of voltage for an extended period of time, extreme damage may be done to the battery. Without Battery Tender® type electronic safety controls, what appears to be an initial cost savings for the charger may actually cost several times the charger price in replacement batteries


Yeah, the one that I got does the exact same thing, without the marketing BS... Again, read my first post.
 
#17 ·
i stand by my post
and i don't see where the china unit you listed is a smart charger ?
not that i care but i did read scan the spec in the link

i didn't think you would appreciate my post

but it was just to show others an easy option with a warranty and accountability

I like DIY stuff I have way more than most people
but with electronics like this that can wreak havoc on your truck I prefer my choice
 
#18 ·
I have that same battery charger.

However, before buying that charger I used to use an old Linksys router wall adapter to charge my motorcycle battery. One day I tried to measure the amps put out by the adapter using a multimeter and my adapter went dead. The multimeter still works fine.

Can someone tell me if I went about this the right way?

Here is the diagram I used:

Product Technology Electronic device Electronics Electronic component
 
#20 ·
For current, he would need to be in series.

Can someone tell me if I went about this the right way?
Are you sure that you had the meter leads plugged into the ammeter position on the face of the meter and not the volts-ohm position (where they stay almost constantly on just about everyone's meter)? That is the only thing that I can think of short of some other condition not conveyed by the diagram.
 
#24 ·
Once the shock of the purchase is over the 4 station battery tender I have is pretty darned sweet, both Motorcycles and both trucks are always hooked up and being tended when I'm not behind the wheel (or bars). My factory batteries lasted 11 trouble free miles

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#30 ·
Stoopid smartphone, yes years not miles, could swear I typed miles.
That's my story and I'm sticking to it

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#28 ·
The problem with that charger is 12V output. A 12V lead acid battery is not "full" at 12v, in fact it's only about 25% charged. If left at 12v for any length of time the battery will begin to sulfate. It's a sure way to kill a good battery.
 
#29 ·
The charger that the OP mentions transmits no voltage when connected directly to multimeter leads. However, when connected to a battery........it does.

Without the charger connected......my battery was at 11.97V. When I connected the charger, the battery voltage went up to 12.17V.

It only took 4 to 5 hours to bring the battery voltage to 12.80V It wasn't like it was connected for a long time for sulfation to occur.
 
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