I have a 12v lead acid marine battery that powers my backup sump pump. I have a trickle charger connected to the battery. Once a month or so I manually run the 12v pump for a cycle or two. This has worked just fine for the past few years. Now I would like to power a rasberry pi + sensors from the 12v battery, using a 12v->5v power adapter (probably just a cigarette lighter USB adapter). I will probably have to put a larger charger on it. I believe I should still use an automatic charger/battery maintainer, but the output must at least equal my draw from the raspberry pi + sensors of course.

I will be switching from a .5A harbor freight battery maintainer to this: (because I already own it) https://www.walmart.com/ip/BLACK-DECKER-BM3B-6V-and-12V-Battery-Charger-Maintainer-BM3B/21721663?

It will charge at 1.2A. Sound ok? Do you see any problems with this plan? Thanks

 

I will be switching from a .5A harbor freight battery maintainer to this: (because I already own it) https://www.walmart.com/ip/BLACK-DECKER-BM3B-6V-and-12V-Battery-Charger-Maintainer-BM3B/21721663?

It will charge at 1.2A. Sound ok? Do you see any problems with this plan?

Looks okay to me, but why are you switching from a 5A charger to a 1.2A?

 

Looks okay to me, but why are you switching from a 5A charger to a 1.2A?

.5 - half an amp.

 

.5 - half an amp.

Darn.
Pesky little decimal point. ;-)

 

1.2A may not be enough. See this.

 

1.2A may not be enough. See this.

Yes, from that article - The maximum power the Raspberry Pi can use is 1 Amp

That's 1 amp at 5v. I'll be using a 12v->5v USB adapter like this. I assume the current on the 12v supply will be less (75%?) of the 5v current draw. But you're right it might not be enough. I was thinking of using INA219 current sensor to keep an eye on charging current and the battery voltage.

My additional sensors will probably draw less than 50ma total.

 

I assume the current on the 12v supply will be less (75%?) of the 5v current draw.

I would think it would be no more than 50% (80% efficiency), otherwise the adapter would get pretty warm.

 

And I wonder what if you disconnect just the battery from the circuit and leave the charger connected? Will it damage the rasberry or sensors?

Sorry to be quite off topic, but I don't know why I cannot power up modern smartphone without a battery, even with charger plugged in. Sometime I wish to test for damaged battery, without first having to buy a new one.

 

I would think it would be no more than 50% (80% efficiency), otherwise the adapter would get pretty warm.

I just did some tests, the 12v current consumption is about half of the 5v consumption. So 450-500ma on the 12v in and 900ma on the 5v out. Unfortunately the voltage on both of my 12v USB chargers drops to 4.6v at 900ma, which is borderline low for the raspberry pi. And on one charger it continues to drop as the charger warms up, despite being rated for 1A output. I accidentally toasted the other one hooking it up backwards, so I'll need to find a new 12v->5V usb charger.

 

And I wonder what if you disconnect just the battery from the circuit and leave the charger connected? Will it damage the rasberry or sensors?

Sorry to be quite off topic, but I don't know why I cannot power up modern smartphone without a battery, even with charger plugged in. Sometime I wish to test for damaged battery, without first having to buy a new one.

Some phones can, but it's probably not a good idea. The batter acts as buffer for the charger.

You can test the battery on it's own with some discharging equipment. Roll your own with a dummy load and amp hour meter or use something like this.

 

I tried a number of 12v->5v USB car chargers. Most performed poorly and would drop voltage to unacceptable levels after 10 minutes or so.

The best one I tried is this one that costs less than $1 on Ebay. It maintained 4.85v at 1A for an hour test. And was not hot to the touch, and even consumed less power than the others (375ma at 12v). Probably because it's a switching converter and I'm guessing the others use a regular voltage regulator. It's rated for 3A but you need active cooling to get that much out of it.
https://www.ebay.com/itm/DC-DC-12V-...Step-Down-Converter-Module-6-24V/163779904027

 

I tried a number of 12v->5v USB car chargers. Most performed poorly and would drop voltage to unacceptable levels after 10 minutes or so.

The best one I tried is this one that costs less than $1 on Ebay. It maintained 4.85v at 1A for an hour test. And was not hot to the touch, and even consumed less power than the others (375ma at 12v). Probably because it's a switching converter and I'm guessing the others use a regular voltage regulator. It's rated for 3A but you need active cooling to get that much out of it.
https://www.ebay.com/itm/DC-DC-12V-...Step-Down-Converter-Module-6-24V/163779904027

If a linear regulator were used, then the 12V and 5V current drains would be essentially equal (ignoring a small bias current for the linear reg device). Your measurements of different currents indicates that the chargers use switching regs of some kind.

 

I'm guessing the others use a regular voltage regulator.

Not likely, as that would dissipate 7W for a 1A output, which would fry the electronics and melt the plastic package.