I'm in the market for some over-discharge protection for a 12V battery bank and was wondering what you'd suggest.
The application in this case is for a solar-powered server cluster. The cluster consists of 5 servers (8-core Intel Atoms), a battery monitor PC (ARM), border router (PC Engines APU2), an ADSL modem and a 24-port gigabit switch. The whole lot runs direct off two 105Ah AGM batteries wired in parallel. No inverters, the servers run off Mini-box PicoPSU power supplies with Micrel MIC29712 LDOs, the 240-12V PSU in the 24-port switch was also replaced with a MIC29712, the APU2 runs off a LM2940CT-12, the ADSL modem runs of a LM-2596 adjustable switch-mode PSU set for 5V and the battery monitor PC, being an industrial-rated PC is happy running directly from 9-30V DC.
On the roof are 3 12V 120W solar panels wired in parallel to a fuse box under the roof of our back deck using 8GA figure-8 cable, and from there it goes to 6GA figure-8 cable approximately 10m to the solar controller.
Detail can be seen at https://hackaday.io/project/10529
In the beginning, I ran a Redarc BCDC-1225 controller; I had the change-over relay set up so that I could either run it off a 12V 40A switch-mode PSU (Powertech MP-3809), or from solar panels, however I've found that while the BCDC-1225 will happily run off either if powered down prior to switch-over, it isn't so good at switching from solar back to mains "live".
I ran it a little while purely as a solar controller with a Xantrex charger, but the Xantrex had a fault and cooked the now damaged batteries. (Come home 2AM one morning, open the garage door… last thing you want to be thinking at that hour is "why do I smell sulphur?")
I've since procured a Meanwell HEP-600C-12 which is doing the mains charging, whilst a Powertech MP-3735 is managing the solar. The Meanwell does a good job, and can be controlled from a 5V TTL signal to boot, so I have a small ATTiny24A controller monitor the battery voltage and it turns the mains charger on when the battery gets low.
The MP-3735 though, has been utterly hopeless. For comparison, this is the battery voltage throughout the day when the BCDC-1225 is hooked up:
On a similar mostly fine day, this is the MP-3735 (I've simply disconnected the solar input on the MP-3735 and hooked that up to the BCDC-1225 -- I have SB-50 connectors throughout).
So I'm looking at giving the MP-3735 the boot and putting the BCDC-1225 back in permanently.
Now, the only thing the MP-3735 offers me right now, is that it has built-in over-discharge protection. My load hovers around the 8-9A, but can spike over 10A. For the sake of being conservative, let's call it minimum 20A … anything connected in series between the batteries and the load must be able to carry 20A continuous. I need to cut power when it gets below about 11.8V (much below that, and the IPMI BMCs on the servers will moan that their 12V rails are sagging).
Jaycar do have one that looks promising, but I'm also aware that this is the same place the MP-3735 came from and this is a single point of failure.
It isn't difficult to rig up a little LM311N comparator to a beefy SSR, or make a small microcontroller handle switching a SSR, but I was wondering if there was a suitable over-discharge protection system that might do the job.
As always, I'm open to suggestions, and if there are queries about my set up, I'm happy to answer them.