Battery Charging

[DavZeeMXT]

I’ve got a “Battery MINDer” charger and wondering how others hook up to their batteries. My MXT-7 battery is located under the cowl against the firewall. Is it possible to charge through the cigarette plug? Do you have some sort of other easily accessible plug that doesn’t require lifting the cowl?

[andy]

I have a trailer hitch type cable installed under the copilot seat to the battery solenoid through an inline fuse. The connector is one of those male and female molded plugs that is impossible to reverse polarity on and has a cover that protects the terminals when not in use.

[wtxdragger]

I use the plug assembly that came with my Battery Minder.
My battery is behind the rear baggage panel and my plug lays on the floor of the baggage area. I can plug in the charger and still close the baggage door over the cable. I would like to have a bulkhead plug installed through the fabric just behind the door pivot area.

I would also like to put in a good word for the Battery Minder, mine works flawlessly.

[andy]

Cigarette lighter plugs are usually wired to activate when the master battery switch is turned on. Unfortunately, that also turns on a lot of other things that will drain the battery. You could have the cigarette lighter plug rewired directly to the battery through a dedicated circuit breaker and fuse but you would have to remember to unplug anything connected to it that would drain the battery. I think it would probably be better to install a separate charging cable wired to the battery solenoid through a fuse.

[DavZeeMXT]

Thanks Guys, I’m going to use the plugs Andy suggested and run it to the battery solenoid. I’ll be able to access it through the oil dipstick hatch on the cowl.

[chris_01]

anything against connecting the charger the jump-start cable to the external plug between RH door and cowling?

[andy]

According to my parts DVD drawing 7203F the external Aux Power Plug connects to the D.C. Main Bus through a 50 amp circuit breaker. That means that the Battery Main switch must be turned on for it to activate the Battery Contactor and charge the battery.

If you can turn off everything connected to the D.C. Main Bus so nothing will drain the battery while you charge it, then you can connect something like a battery minder to the external Aux Power Plug to charge the battery. Most battery minders provide around 1 amp to charge the battery at a fairly slow rate so they can't handle any components connected to the D.C. Main Bus that draw a significant amount of current.

In my case I have a RC Allen 26 series electric attitude indicator that draws 2.4 amps starting current and 1.2 amps running current connected to the D.C. Main Bus through a non-pullable circuit breaker. It doesn't have a power switch so I can't turn it off when I switch the Battery Master switch on. I don't have an external Aux Power Plug but if I did, then I wouldn't be able to connect a battery minder to it and charge the battery while running the electric attitude indicator.

If you have a configuration that allows you to turn off everything connected to the D.C. Main Bus using each component's power switch, then you can switch on the Battery Master and connect a battery minder to the Aux Power Plug without a problem.

In my case it's much simpler to connect a separate cable and fuse to the Battery Contactor 2B1a post (the positive side of the Battery Contactor coil, which is jumpered to the + battery post) and connect the battery minder to it. Then I don't have to turn on the Battery Master to charge the battery and I don't have to remember to unplug stuff from the cigarette lighter receptacle. When you wire the battery minder this way, you have to find a good ground near the battery or Master Contactor. In my airplane they are both under the copilot seat. You can't use the ground side of the Battery Contactor since it's only activated by the Battery Master switch.

That raises a point that you should use pull-type circuit breakers as much as possible, especially if you have an electrical component that doesn't have its own power switch. Even components that have their own power switches should use a pull-type breaker when possible since internal power switches can sometimes fail mechanically and prevent you from turning them off in an emergency.

[DavZeeMXT]

Very helpful information Andy. You saved me a problem because I didn’t know about the negative side of the contactor only being on with the master. Shouldn’t be too hard to find a suitable ground somewhere in the vicinity?

[andy]

DavZeeMXT, I will try to remember to take a photo of where my ground point is next time I'm at the hangar and post it here. You shouldn't have too much trouble finding a good ground. Maybe on the battery box. If you have an Ohmmeter, set it on the lowest scale and measure between the negative terminal of the battery and candidate ground points. The lower the reading, the better the ground. Whatever point you pick, make sure it's copper, steel or a good metallic conductor and that it doesn't have corrosion, paint or movement. Aluminum is not a good conductor.

[Andy Young]

The mounting bolts for the main relay are excellent grounds. And handy.

[cooker]

I would suggest a light fuse on the + battery minder cable close to the battery post. Leads could short out especially where they are routed under bulkheads or where door closes on them. Had it happen with a low quality trickle charger, not real sure if leads were pinched , poor quality or possibly cracked from cold weather but it really made a mess and would have been a disaster if we would have left the hangar before noticing what was going on.

[Kirk]

Recently found this solution from BatteryMinder. Approved for certificated aircraft. Fuse included, “aircraft grade” wiring.
Says logbook entry only by A&P.

https://www.batteryminders.com/batterym ... -aircraft/

What do you guys think of it?

Kirk

[andy]

The BatteryMinder BM-AIK2A looks like it would do the job. The only thing unique is the molded quick connect plug/receptacle. Otherwise, a regular trailer hitch connector would work just as well. I liked the insulated alligator clips that can't be shorted together but most aircraft will use the ring lugs connected to the battery contactor (solenoid). As cooker said, one end of the inline fuse holder should be mounted as close as possible to the post on the battery contactor to minimize the length of unfused wire for safety reasons.

If you use a trailer hitch connector like this, the fuse holder is close to the battery contactor post and is in a rugged, waterproof housing. It uses a blade type automotive fuse, which is much more rugged than the typical glass enclosed fuse, inexpensive and widely available. You can buy this one from Amazon for $9.95 which comes with two assemblies: https://www.amazon.com/Tetra-Teknica-Mo ... 838&sr=8-8. The ring lugs are already crimped with a professional grade crimping tool and it has a 10-amp fuse. This eliminates the need for inline butt splices between the wire and a separate inline fuse holder. Butt splices are notorious for introducing intermittent connections since most people don't use good crimping tools. Notice that the end cap on the male connector exposes only the negative pin if accidentally left off the plug. That's for safety so you can't accidentally short the positive lead against a ground. I can't tell if the ring lugs are large enough to fit the post on the battery contactor but you could cut them off and crimp larger ones on if needed.



Even if you use a regular trailer hitch plug/receptacle, you should only need a logbook entry from an A&P. It's a minor airframe alteration. I've encountered some A&Ps who get nervous about minor alterations if they are not on a STC or explicitly described in Part 43. I know one of them who called the FSDO to ask how it should be handled. They told him that it was similar to the way a tablet EFB is wired into the aircraft electrical system. That made him comfortable enough to sign off. It's really not that similar since no circuit breaker is required. The fuse protects both the wire and the BatteryMinder.

[DavZeeMXT]

Thanks for the link. That plug harness is going to work great for me.