Ground power options

[langted]

I am in a flying club with a nice low time Piper PA28 140, and we just upgraded it with with a Garmin GPS175 and Aspen E5 (eliminating some old stuff, including vacuum system).   Being in the US, it's still Winter, and usage is low.  However, our 8 semi-geriatric members (including me) will very soon be needing a ground power supply, so as to have a bit of mercy on the battery while getting comfortable with the new equipment.  (To be honest, I could also use it for a little play time and currency in my own glass panel Arion Lightning as well.)

 

My thoughts are to buy an RV (that is, Recreational Vehicle) "power converter" as these are readily available at less than about $120 US (or $190 AUD) with generous capacity of 45 to 60 amps and a piper style plug.   In general, the modern RV power converter units are switching style power supplies, advertising low ripple that is suitable to drive electronics with or without battery filtering.   They also double as battery chargers, and can be connected long term as long as they are bought for the type of battery in use (flooded wet, AGM, lithium).   In general, these have a sag in delivered voltage, so that if connected to a weak battery, they don't charge at ridiculous currents.   That would seem to be safer than, say, a straight power supply that regulates to simply hold a fixed voltage of say 13.8V.   I would think a simple regulated bench power supply might not be so great to connect to small semi-discharged aircraft battery that isn't too keen on being hit with, say, 60amps.   

 

If an RV power converter unit sags in voltage to, say, 12.5V or so while in use, it seems to me that a standard battery would just give up a little charge, but be replenished once the training session is over.   

 

An alternative would be something like the more pricey Aircraft Spruce 12V, 25A power supply with Piper plug (11-05086), currently costing $289 USD.   However, although Aircraft Spruce lists it for use as a ground power unit, they also state the power supply is a Schauer model JAC2512-PC.   If I go to the Schauer web site, they say it's a battery charger and not for use as a power supply.   I'm guessing that's because it needs a battery to smooth out power, but I really don't know why they don't recommend it as a power supply.  That plus the much higher cost made me look for something else. 

 

What do you guys recommend ?   

[facthunter]

A bigger  or several batteries in a cart kept charged  by its own charger and plugged into your ground socket when needed. IT cannot hurt your system. with big battery capacity  smoothing things out. leave the charger off if you are worried  some of the Modern electronic ones have Pulsed voltages. Nev

[3rd harmonic]

Those RV wall power supplies are much like a 13.8V bench supply with some current limiting at maybe 20-40A. But a 13.8v bench supply would be fine anyway, you ain't going to get 60A flowing even if can do it... Essentially, if the battery is in a reasonable state of charge at 12.5V, even 0.25ohm on a 1-2m of your connection cable to the battery which could be 8-10 AWG will limit the current into the battery by (13.8-12.5)/0.25 = 5.2A

It will gulp abit of current initially, after some minutes it will push the battery volts up to the float level (13.8v) and the draw will be minimal, plus whatever is turned on in the plane.

[Kiwi]

8 hours ago, langted said:

piper style plug.   

I think you will find that the piper external plug does not send power to the battery, you can not charge the battery through it. It only goes to the starter relay for jump starting. 

[Underwood]

Kiwi surely the relevant relay connections go straight to the battery?, so this would work?

[facthunter]

No point in speculating. If does or it doesn't A lot of ground power units do power the aircraft but the choice should be by switching, so that you know EXACTLY what's going on.  Nev

[Underwood]

Yes sorry, I was  curious for myself as all  solenoids I have seen have the heavy duty starter cables direct to the battery and it would be these points on the solenoid that a jump start connection would be made to, perhaps there is a further heavy duty relay in the line that needs switching?

 

As you said, It is a question that Langted needs answering before connecting anything

[Kiwi]

1 hour ago, Underwood said:

Kiwi surely the relevant relay connections go straight to the battery?, so this would work?

I just google the piper external plug and got some interesting results, 

No, you can't charge the battery via the external plug, the master switch must he on for external power to make it to the battery. 

So yes, if you are testing out new avionics (the master is going to be on), it should work. 

 

Here is an interesting link about it.

 

https://piperowner.org/talk/discussion/154730/external-power

 

ps, I own a PA-28 140. 

[Kiwi]

[langted]

Thanks for responses.  Indeed the port on the Piper goes straight to the starter solenoid.  You have to turn on the master,  but once the master relay switches on (using onboard battery power), the outside port is connected to the battery and all the loads. Odd way to wire ii, but I suppose in 1967 there was less need to just sit and play for hours trying to learn the features of an avionics set.  
 

So, if one wanted to use this port for a maintenance charge, it clearly won't work since the master relay itself poses a load that would screw up the charger capacity and voltage sensing.  But for learning the avionics, it'll do.  And it'd be ok for a bulk charge.  
 

So back to the original question, i have options of a large bench power supply, large battery charger, RV "power converter", or a big enough battery on a cart.  Not having a big battery available, I'm leaning to the RV power converter for cost and flexibility.   Any reason it would not work, damage something, etc?  

[3rd harmonic]

9 hours ago, langted said:

 

So, if one wanted to use this port for a maintenance charge, it clearly won't work since the master relay itself poses a load that would screw up the charger capacity and voltage sensing.  But for learning the avionics, it'll do.  And it'd be ok for a bulk charge.   

Some understanding of contemporary 3-stage lead-acid battery charging might help:

 

1. Bulk stage: (constant current) the charger pushes the voltage higher at the max current it can source (or is selected) 5, 10, 20A ect

 

2 Absorb stage: (constant voltage) when the voltage gets to 14.2-14.5v, rather than pumping out max amps to push the voltage higher, the charger holds the battery voltage constant. The current into the battery should taper off as the charge is absorbed though ALL the material on the plates.

 

3. Float or maintenance stage: (constant voltage) generally after a perset time at the Absorb level the charger will drop the voltage back to around 13.2-13.8v, the current into the battery should be pretty low at this point. Any additional load across the battery can then be supplied by the charger while the battery is 'floating'.

 

For lead acid batteries temperature strongly affects the charging end point voltages and this needs to be compensated for - correct charging of larger battery banks in fixed installations generally always has a temp sensor on the cells.

Especially for batteries that are not used all the time, it is IMPERATIVE that they are charged to the absorb level regularly to avoid the plates becoming 'suphated'

----

To answer the question, a bench supply or an RV power supply will just float the battery and supply any load with master switch on which will work fine, although without any absorb stage.

The regulators on traditional aircraft alternators are pretty low tech, generally they only float charge at no more than 13.8v and there isn't any basic temperature compensation that most automotive ones have featured since the 1980s. This tends to result in undercharging, suphation and early battery failure.

Edited Sunday at 03:16 AM by 3rd harmonic
Typo

[facthunter]

You need the Latest type of charger with microprocessor technology, that you set for the Battery type. (NOT including Li Po etc) If your aircraft battery is completely isolated with Master off it shouldn't discharge but if there's a frost that will knock it down significantly,, say 15%.  Repco have a model RBC8SS3 6v/12v and a bigger 12v only. The bigger one can reinvigorate a sulphated battery in most cases. Neither can be hooked up wrong. It reads ERR.  and they can be left on for days without risking the battery. 150$ and  200$  respectively.  retail.  Using this sort of thing will extend  battery life and your flying safety significantly.  Nev