Battery to Battery charging question

[strongmanmike (Michael)]

Ok so I have a great 12V solar battery system working very well at the observatory

See a picture of the power setup HERE

And I just got a Skywatcher 12" GOTO dob seen HERE outside at the observatory

I want to use a smaller 40ahr SLA battery (visible on the shelf in the photo of the battery setup above) to power the Dob outside.

My question is: When I leave of a night, is it safe to simply connect the smaller battery to the two larger batteries (in parallel) and have it charged and maintained with them and leave it unattended in this way?

When the Sun is up the solar regulator bulk charges the big batteries at 14.6V and when they are full floats them at 13.7V (set per battery spec sheet)

Or should I consider using a 12V DC to DC charger instead? Like THIS one

Cheers for any feedback

Mike

[glend (Glen)]

If you have used the dob battery there is likely to be a potential difference between your battery bank and the dob battery. This mean when you do connect them they will equalise, maybe with a spark. I have a similiar setup with a solar charger feeding the battery bank in my shed. To avoid any night time drama with cable connections I simply charge my scope battery the next day off the solar system usually by just running my AC charger off the bank inverter. This smart charger will manage charging the scope battery and go to float when finished. But hey it will work if you just connect them but be aware of the potntial difference and that they will pull down as they equalise.

BTW I have the same solar regulator (30A in my case). Do you have a fuse on the bank? It doesn't look like you are running the load off the regulator so you should have protection between the bank load (ie the inverter pictured) and the bank. I would also suggest a marine battery switch so you can isolate the bank from the load.

[strongmanmike (Michael)]

Cheers Glen

Thing is I am usually not there the next day to supervise as my observatory is on a friends property ie I don't live there (no mains power). So I wanted to be able to just connect the seperate smaller battery to the big bank when I leave and be confident that if I am not back for a week or two I don't ruin the system or worse, melt or burn down the shed

The smaller battery is almost certainly likely to have less charge then the big bank when I connect them. I was thinking it was probably not really an issue but given the smaller battery is not an AGM style battery like the big ones and it's older plus the ideal charging specs of the two different batteries may be an issue too ie the big batteries need to be bulk sharged at 14.6V and thisi may not be good for the smaller battery..? So I was worried there may be some issue connecting them hence why I was thinking the DC to DC charger...?

Yes my solar regulator is also 30A and no the load is not running of the regulator but connected straight to the batteries via the inverter, I think the regulator manual said a fuse wasn't really necessary..?

Mike

[glend (Glen)]

I would not try to put different battery types on the same bank, the charging characteristics are different - especially if its unattended. You could run a DC-DC charger off the load side of your regulator, that way it woukd be running through the regulator load fuse, and charge the small battery that way. Another alternative would be to get a small regulated solar panel and simply clip it to the small battery, but that would be more expensive. You can connect more than one regulator to the same bank so if you had a switch you could switch it between the bank and the dob battery but you would need to be there to do that. The most cost effective solution ( like $20) woul d be to put a 12V regulator on the load side of your controller and it will charge you dob battery and maintain float isolated from the bank and fused.

[strongmanmike (Michael)]

In the end I just got 2 X 5m 12V extensions with cigarette connectors and connected them together and ran the scope straight of the two big Kickass batteries back in the shed, a great all night session of viewing and imaging

The 12" Skywatcher from Astroshop is a great bit of kit. After a two star alignment the bloody thing put every object in a 10mm eyepiece!! and in the steady early hours before dawn all 6 main stars in the Trapezium of the Orion Nebula were cleanly separated pinpoints..very happy

Mike

[rustigsmed (Russell)]

nice job mike, you're not wrong about the 12" SW goto either, you should have a crack with imaging with it! would love to see what a cooled ccd could do vs a dslr (my latest dslr effort - although its no longer in use for imaging

https://www.flickr.com/photos/803366...ream/lightbox/

actually, your setup is pretty much what i'm aiming for, a nice dob to do some visual while the imaging gear is slaving away in a dome. a dome and the property that goes with it is probably about 5 years away yet

[strongmanmike (Michael)]

Excellent M16 Rus! great job, is your SW 12 on a Equatorial mount?

Yes, having such a facility is indeed fantastic, I would love it if it was fully automatic and remote so I didn't have to pack teh car and drive out ect but hey, we canna have everything can we? Good luck with your plans mate

EDIT: Ok I see from your flickr pages you are indeed using it as a dob...even more impressive!

Mike

[rustigsmed (Russell)]

Originally Posted by strongmanmike

Excellent M16 Rus! great job, is your SW 12 on a Equatorial mount?

Yes, having such a facility is indeed fantastic, I would love it if it was fully automatic and remote so I didn't have to pack teh car and drive out ect but hey, we canna have everything can we? Good luck with your plans mate

EDIT: Ok I see from your flickr pages you are indeed using it as a dob...even more impressive!

Mike

cheers mike just goes to show its tracking is pretty good going for just a simple two star alignment.

yes remote hasn't entered my mind - way too many steps away, I guess if you were considering it you'd have to move away from the newt to a refractor /RC / cassegrain for collimation reasons

[strongmanmike (Michael)]

Originally Posted by rustigsmed

cheers mike just goes to show its tracking is pretty good going for just a simple two star alignment.

yes remote hasn't entered my mind - way too many steps away, I guess if you were considering it you'd have to move away from the newt to a refractor /RC / cassegrain for collimation reasons

Actually the AG12 holds collimation and focus pretty well, just last session after two weeks I fired up the scope and goto'd my new target for imaging and the focus was almost perfect When I recently moved the AG12 and set it up at the new location it required only minor collimation tweeks and that should do for the next year I recon

Mike

[GrampianStars (Rob)]

Originally Posted by strongmanmike

Ok so I have a great 12V solar battery system working very well at the observatory....

I want to use a smaller 40ahr SLA battery..... to power the Dob outside.

My question is: When I leave of a night, is it safe to simply connect the smaller battery to the two larger batteries (in parallel) and have it charged and maintained with them and leave it unattended in this way?
......
Cheers for any feedback
Mike

Just run a seperate small 7.5Amp solar regulator direct from panels to the 40Ah battery

[strongmanmike (Michael)]

Originally Posted by GrampianStars

Just run a seperate small 7.5Amp solar regulator direct from panels to the 40Ah battery

Cheers Rob, yes an idea for sure but for now running straight off the 300ahr's worth of onsite solar regulated batteries via direct extension wire connection works well.

Mike