MightyMite DC Power pack Mods
On Saturday (March 2003) I was down at the local Sears. The MightyMite Automotive Jump Starter and DC Power Source, was on sale for about $36. This is one of those emergency power jumper boxes with big battery jumper cable type clips on them and a lead acid battery inside. At $36 we are talking about a good price for a nice sealed lead acid battery, much less a complete rechargeable packaged unit. Regular price is closer to $56.
These emergency jumper packs are attractive because they include a AC charger and float unit and are thus a self-contained portable 13.8 volt [12 volt] DC power source. The MightyMite is a small version and quite transportable. Most of the gnarlyness is the heavy wires and big cheapo battery clamps stuck on it. The idea is that they can be used to jump start a car. If one decides they are just not needed for hooking up a QRP rig, then the unit can be considerably cleaned up.
Once the stupid plastic battery clips are removed by opening the box and getting them out of there, starting by disconnecting them all they way back at the battery terminals, this is a very slick looking unit!
The 8 gauge cables and clips are just an accident in progress and serve no useful ham function I could see. You can also cut off the plastic tabs the clamps are secured to the outside of the case with in the storage position. Giving you a compact package. As the picture shows, they take up considerable space as well as being a short circuit waiting to happen. Between the two clips you can see a 15 amp circuit breaker that is included. The clamps are as they come out of the box, hooked over some slips of cardboard that are folded over the plastic paddle like prongs that stick out the back of the unit. After the modifications these are removed with a modeler's razor saw. The ruler shown is a bit less than six inches, so you can see this is not a big unit at all.
Inside the plastic case is a stock 12 amp/hour sealed lead acid battery and a charger circuit with float. On one end is a pair of prongs you are supposed to shove a heavy duty extension cord female end onto for charging. On the other end is a cigarette car lighter puka. Radio shack makes a plug for this with a built in fuse and cord. So I do not see that any better DC connector is really needed.
There is even a nice rubber lid for the cigarette lighter puka. I ordered some Andersen Power pole connectors so I could put one on the cigarette lighter plug's cord, as that is becoming a standard for portable emergency power usage. I ordered these parts from WestMountain Radio.
The plastic case is held together with a number of self taping long screws into plastic so you do not want to open it more than once or twice. Use care removing and replacing these. Most of them are rather long.
The 8 gauge cables are secured inside with cable ties and run back to the battery where they are secured with large lugs and a bolt along with the charger wires. I just cut mine where they came out of the case and slid the battery out of the half shell of the open case to unbolt them from the battery.
Be CAREFUL! That battery is charged most likely and ready to make quite a show if you short it with the metal tools (experience speaks). True the box and the tag on the unit say you have to charge it overnight before using it, but the battery is ready to rock and roll, or mine was at least, when a screwdriver went astray inside.
All that really needs to be fiddled with is the two battery terminals and all you have to do there is get off the two big lugs for the external battery clamps and replace the wires and bolts. I used a small pair of wrenches.
The case can be a fight to get back together. I had a fair amount of trouble getting the AC input seated just right so that everything was snapped up tight ready for the screws to be replaced. The picture shows the prongs on the end opposite the cigarette lighter puka. This is not a bad arrangement vs. having wires dangling about that have to be coiled around the unit somehow. Plus the AC cord can be quite short if you like. I have one only a foot long which is a bit short, but 3 feet is a good size for a recharge extension cord you would want to lug around with you..
The inside picture at the bottom of the page was taken after the big lugs and the 8 gauge wires had been removed from the battery terminals. It is all ready to package up again at this point. The main circuit board is fastened to the front behind the wiring shown. It has some nice indicator lights on it where you can check the charge level.
After you manage to get the clips completely cut off you end up with a unit that is very transportable. You can see the cable tie that prevents the wire from pulling out on the chopped up remains in this finished picture. Also the front panel with its red battery testing light and the associated LED's. The handle is comfortable for carrying and the rounded shape makes it pleasant to move around as opposed to sharp pointy corners to jab the legs, etc.
The final result is not that heavy. With the big battery clips gone it is significantly more compact. With them and the plastic tabs cut off, it is a good form factor with few snaggy bits sticking out. The cigarette lighter is a workable connector since mating plugs with fuse are available and we are not looking at high amperage connections here. For most uses with QRP and HT's you will be sucking less than 4 amps even with an SG2020 running power. You probably do not want to drain more than 3-4 amps from a 12 amp/hour battery anyway. For more than about 4 amps other than very short periods of time, you should just get a bigger battery. There are larger versions of these jumps start portable units. Some even include fancy stuff like 120 volt A/C inverters built in. Of course they cost more and are heavier. I wanted the smallest, lightest possible unit that would power a small HF or VHF station for a reasonable outing.
If one figures the typical power drain on receive, and 50% duty cycle on transmit, one might expect the battery to last five or six hours if maximum transmit drain was 3 amps or so. As a general rule you would not want to drain a battery faster than about 1/3rd of its rating in amp/hours, and for full rated discharge life, closer to 1/10th is better. And if one wants to get full amp/hour rating out of a battery, I have been told that closer to 1/20th of its rating is more reasonable.
As you can see, this is a rather nice looking battery. The AC connector package is on the left and the DC cigarette lighter package is on the right. In the middle is the resettable circuit breaker. My biggest problem with this mod was getting that AC connector module settled back into its spot in the two case halves so they could close firmly and snap all the way down before reinserting the screws.