Battery testing

With a new mount, I have a couple of visits to dark sky sites with no electric utilities ahead. Beefing up my battery options is necessary. I have 2 batteries that I've used to power my CPAP (with poor results) and to run my Vixen Polarie astro rig (great results) all night. Replacing my tiny Vixen Polarie with a Sky-Watcher AZ EQ5 Pro was certain to demand a more capable batter. I decided to try something new.

The batteries

I'm an electrical engineer, not usually interested in the big consumer power banks that bundle a mystery battery, a fuse, and a plastic case for 2 to 3 times the cost. I package bare batteries in a small box with a fuse, outlets, and the charger myself. Here are the high capacity batteries I have now:

  1. A standard sealed lead acid deep cycle 12 V x 12 AH used for wheelchairs and carts.
  2. A Talent Cell Li-ion 12 V 142WH (~12 AH) battery. A nice packaged consumer unit, much smaller and lighter than lead acid batteries.
  3. A Miady LiFePO4 12 V x 16 AH battery that I recently bought to check out the technology.

The box on the left has my sealed lead acid battery. The one on the right the LiFePO4 and lithium batteries. All with the needed chargers.

LiFePo4 batteries are small and light, but there is much less risk of fire than with Li-ion. They have a relatively high and very flat discharge curve and integrated battery management systems that shut off output to save them from deep discharge damage. They are smaller than the same capacity lead acid battery and half to one third the weight!

Informal use testing

An independent test ($54 12V 16Ah LiFePO4 "Miady" Pack Tested )found that the Miady ran about 20% below it's rated capacity. However, they were selling for about 1/3 the cost of other LiFePO4 batteries for a while on Amazon. I Normally LiFePO4 batteries are quite expensive compared to other types. Probably due to the BMS electronics and early adapter economics.

I've had poor experience with both the lead acid and the Li-ion batteries with my CPAP machine, which seems to be very sensitive to voltage. Very short run times or failing battery checks on startup.

The Li-ion pack has done a great job running my Vixen Polarie star tracker and Sony mirrorless camera. Easily overnight, about 8 hours is the longest that I've tried. I've run just a mount on the lead acid battery for a few hours in the past.

I decided to jump in the deep end, running everything off from one battery: mount, cooled camera, ZWO ASIAIR pro controller, dew heaters, and router.

The sealed 12x12 lead acid battery quickly ran into trouble after its voltage dropped to about 12.6 volts. Both the ASIAIR and the mount are supposed to run at voltages well below this. The problem was unreliable communications between the ASIAIR and the mount. I suspect that the weak link here may be using a USB to USB cable and that I'd may get better results with an EQMOD USB to hand controller port cable. I didn't think to test that this time.

Next I tried the LiFePO4 battery because of my positive experience using it with the voltage sensitive CPAP machine. Results were outstanding. Voltage quickly dropped to about 12.9-12.8, but no further for quite a while. After focusing and polar alignment I settled into imaging. The LiFePO4 battery ran for just over 5 hours from first image to the last image when the BMS shut down the battery!

The low voltage indicators on the ASIAIR and the mount never activated in either test. The LiFePO4 BMS cuts off power before either. With the lead acid battery, I suspect the mount USB interface fails at a higher voltage.

Future plans

I'm confident I have an all night (8+ hours) solution with the following changes:

  • Don't use the router! There is no need for this at a dark site and it's power draw is significant.
  • Use two batteries: the LiFePO4 battery for just the power hungry mount, and the Li-ion battery for the camera, controller, and dew heaters.

Content created: 2021-03-19

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