Need help with my solar! Is there an automatic battery switcher out there?

Thread Starter

Natdak

Joined Jul 11, 2021
2
I am being offered many used truck batteries for free from a neighbour that repairs trucks at a volvo factory. The batteries are 12v 235ah volvo lead acid batteries that often still have quite a bit of life left. I know that one shouldnt connect dissimilar batteries in series or parallel but is there a device that can automatically switch between batteries when they become full/empty? 4 or more batteries would be perfect and the switch would have to handle 160a, but preferably more.
If such a thing exists at a reasonable cost, I think it has very big implications, not just for me but for the environment, waste reduction and peoples wallets.
Another, less favorable option would be if there is a manual changeover switch with at least 4 inputs and 160a+.
 

Reloadron

Joined Jan 15, 2015
6,220
What you want is a 12 volt continuous duty solenoid. Similar to those used on large engine starter motors. They look like this. Then you use a small rotary switch to select any one of four (or more). You want continuous duty and I would look for ones rated at 200 amp. A simple rotary switch could be used to select which is active just making sure the switch can handle the coil current of the solenoid. All battery commons are tied together as are all solenoid commons.

Auto switching can also be done but gets a little more complicated. They make battery monitors which will drop a battery offline when the voltage drops below a preset set point. This can get tricky in they when a battery voltage drops under load and the load is removed the voltage will increase so they incorporate what we call "hysteresis" in the design so a relay isn't banging away.

If I were to do this today I would just use a uC (micro-Controller) for all of the decision making. The single uC would monitor all four (or more) batteries using analog inputs and using outputs select a battery. This method would require a little code writing but is not all that difficult. The process can also be done using discrete components but it adds to the circuit complexity. Anyway in either method the solenoids are switched On/Off manually or automatically.

Also keep in mind all of the heavy current carrying conductors need to be rated for your maximum current or in this case I would look to 200 amp capacity.


Ron
 

Ian0

Joined Aug 7, 2020
3,766
I am being offered many used truck batteries for free from a neighbour that repairs trucks at a volvo factory. The batteries are 12v 235ah volvo lead acid batteries that often still have quite a bit of life left. I know that one shouldnt connect dissimilar batteries in series or parallel but is there a device that can automatically switch between batteries when they become full/empty? 4 or more batteries would be perfect and the switch would have to handle 160a, but preferably more.
If such a thing exists at a reasonable cost, I think it has very big implications, not just for me but for the environment, waste reduction and peoples wallets.
Another, less favorable option would be if there is a manual changeover switch with at least 4 inputs and 160a+.
Albright is the company for DC contactors
https://www.albrightinternational.com/products/rw180-2/
 

Thread Starter

Natdak

Joined Jul 11, 2021
2
Thanks Ron and Ian! Im not an engineer but all that makes sense to me. Can one have hysteresis in the microcontroller to avoid that problem?
The DC contactors are quite expensive though. Im seeing them at £60 or £70 each which is a little bit prohibitive for me. Maybe I can suck it up if the other parts dont add too much cost. I have a friend that can help me with the coding side of things.
Are car solenoids out of the question? Or any devices that these are likely to be found in? Im a big fan of being resourceful from the things readily available to me, even if it means settling for less sometimes or that it takes a while to gather the materials.
Cheers
Nathan
 

Reloadron

Joined Jan 15, 2015
6,220
Hysteresis in real simple terms is like saying there is light and dark but there is twilight between the two. So what happens is at dusk a cloud reduces the light on a sensor. The controller sees dark but as the cloud moves on minor light returns. The controller sees this and the relay (or other device) changes states. In some cases the relay starts chattering On/Off over and over again. Hysteresis eliminates that problem. Hysteresis can easily be applied in both an analog or digital uC design. A Google of Hysteresis Arduino should give some code examples as to how to do it and likewise a Google of Hysteresis Comparator Circuit should give some examples of how it works in an analog circuit. Simply put between light and dark you don't want your output device "freaking out" over On/Off. :)

Both analog and digital devices normally account for this and if you want to roll your own it's part of design time or your code.

Automotive relays and solenoids are designed for DC and handling high current loads. They are also inexpensive depending on what we consider expensive. Automotive relays can handle 30 ~ 40 amps at 12 VDC and a 12 VDC or 24 VDC coil. You choose your switching device based upon your load requirements.

Ron
 
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