I've been trying to figure something out for a while, and have experience with digital systems (logic gates, flip-flops, etc.), but not necessarily "real" systems (with Zener diodes, etc.) so here goes the preamble...

Looking at other posts here, I'm not going to try to build anything. What I want is something to run my 19V Router (1.75A max based on the wall wart) + 5V 2A ATA (2A, again by the wall wart) during a power failure. But I want hours of runtime, and not minutes, so an off-the-shelf UPS is right out (without any modifications, AC->DC->AC->DC conversions).

Reading a response on this post, it seems like the best idea is to hook up a charger right to an AGM battery (or other lead acid). So then I want to get a good charger to maintain the life of my battery, so I came across a charge controller for solar panels, which is great, because according to this thread, I can attach a 120VAC -> 30VDC transformer to a 24V charge controller, the controller to a 24V AGM, and the battery terminals to a few buck regulators for the 19VDC and 5VDC, and voila! Simple DC UPS... I think.

Now here's the question(s): 1) Should I worry about the charging profile (PWM and MPPT) affecting the buck regulators / the output to the router & ATA? Or will the presence of the battery help "buffer" the fluctuations to keep the steady ~24VDC? 2) This is one I'm throwing out there to see what people have to say about it: I notice that the charge controller has a "load" output (link added to explain what I'm referring to, not necessarily for buying advice). Could I use this instead of connecting directly to the batteries? I doubt I'll need to use the LVD function, but I don't know how it'll work (0 switchover time, etc.). This post makes it look like it doesn't work that way looking at the relay, but I don't know.

Thanks in advance!


Yes, you can use a standard solar charge controller as a battery UPS. You need to feed it a DC voltage within the proper range (and it really needs to be a pretty clean voltage - minimal ripple).

If you are using it to power your router, there is another issue you need to be aware of. Lead Acid batteries (wet, gel & AGM) have a voltage profile. i.e. at 12V battery is not really 12V. A good MPPT charge controller will match it's charging profile to the needs of a batter which may range as low as 11.2V before it turns the load off and as high as 15V when fully charged. In a 24V system, that means your battery will almost always be fully charged and floating around 29.7 - 30V. The charge controller will simply put that voltage out on the load terminals unless it has a built in Voltage regulator. You may need to add a regulator to your system. I recommend the set point on this regulator be below your cutoff voltage (10.5V or 21V) and make sure it's input will handle the highest battery voltage (15V or 30V)

You basically end up with a 3 part system 1) AC-to-DC converter for input 2) Solar charge controller, and 3) Load regulator.

It may be cheaper to buy a purpose built UPS and just put a bigger battery on it.


Designing a quality solar battery charging system CAM be as simple as connecting the right COTS components together - BUT knowing how to design to use the COTS parts can be a challenge in itself. Many sellers tell you that their product does all you need and want. Some are correct.

Any quality battery should use a quality charger.
If Icharge is << I_charge_battery_max and if the battery is not fully charged by solar ever then 'any old system' will work.
If you want your AGM LA to live long and prosper you want a charger targeted at the chemistry and battery capacity concerned.

Some "solar controllers" include good LA battery chargers. DSome don't.

More later maybe, but start with Battery University in general and eg
Their AGM charging related pages

[BU-403: Charging Lead Acid

Learn how to optimize charging conditions to extend service life.](http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery)

[BU-908: Battery Management System (BMS)

Learn about old and new methods and how innovations may change old habits.](http://batteryuniversity.com/learn/article/how_to_monitor_a_battery)

[BU-201a: Absorbent Glass Mat (AGM)

Learn what differentiate AGM from other lead acid battery types](http://batteryuniversity.com/learn/article/absorbent_glass_mat_agm)

Anon ...

LA .... Lead Acid
AGM ... Absorbent Glass Mat
COTS .. Commercial off the shelf
...... if you are lucky Cheap ...


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