Diodes for charging dual battery banks

[chorky]

I must admit I have long forgotten the intricate details of diodes and how to select the right one for the right purpose. Sure, YouTube has great info but I would rather seek an answer here first.

Quick backstory. I have modified my Genesis dual battery kit to having one 94R battery in the stock battery tray (removal of the aux), and the solenoid to control power via a 150a fuse and 4ga cable to two batteries in the bed. The system is also connected/disconnected via two high amp blue sea battery switches. One switch controls which of two house batteries in the bed supply power to the system (or both combined). The other controls what system provides power to the 'house' - ie power comes from aux batteries in the bed, or power comes from starting battery under the hood, or it is all combined together as one large battery bank. I have this specifically for my application in cold weather and for self-jump starting.

My solar charge controller can produce up to 11a with the current 2x100w panels. It is wired currently to only charge the house batteries. I would like to add diodes to split the charging between the house batteries and the starter battery. The purpose for this is to constantly provide a small bit of charge to the starter battery when the panels are producing power.

I imagine needing two diodes. One diode to prevent power back feeding from the starter battery into the aux batteries, and one diode to prevent power feeding from the aux batteries into the starter battery. Only power from the solar controller shall go to the aux batteries and starter battery independently.

Yes I am aware that I can use the blue sea switch to combine them all together and that would effecting do the same thing; however, I prefer to have a system as easy and automatic/automated as possible. My only concern here is specs needed to select the correct diode for the job, and what if any issues would exist with the charge profile to the aux and/or starter battery system by doing this (I don't believe any charging issues would exist as the house system monitors ONLY the house batteries ie the shunt only sees - power flowing through the aux system). But, one potential issue I could see is with a diode in place, will the solar controller be able to see voltage levels? Or does a diode effectively block the ability to even see voltage? In other words, if you put a diode on the end of your positive voltmeter, would the voltmeter essentially show a battery as 0 volts?

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[Mr._Bill]

Wouldn't it be easier to get a second charge controller for the battery under the hood and wire the solar panels so they are feeding both controllers?

 

[Pliny]

Wouldn't it be easier to get a second charge controller for the battery under the hood and wire the solar panels so they are feeding both controllers?

Unless there isn't room under the hood, do that.

A diode is going to have a roughly 0.6 V drop across it when it is forward biased. Your charge controller is going to be trying to get your battery to a certain voltage (hopefully based on the type of battery chemistry that you have) during each phase of the charging / maintenance cycle. Certainly seems as if it would result in the batteries not being fully charged and potentially shortening their life cycle.

 

[chorky]

Wouldn't it be easier to get a second charge controller for the battery under the hood and wire the solar panels so they are feeding both controllers?

Yeah I did think about that. And the charge controller is under $80, surprisingly cheap. But it's not waterproof so putting it under the hood is a no go. I could put it in my bed along with all the other electrical stuff, but I would have to rearrange it - for the 5th time.

But the bigger concern I would have is what that would do in regards to the solar panels. Can you have a ban of solar panels successfully running two different charge controllers? I have never seen anything hooked up like that before. But it's worth digging into.

Unless there isn't room under the hood, do that.

A diode is going to have a roughly 0.6 V drop across it when it is forward biased. Your charge controller is going to be trying to get your battery to a certain voltage (hopefully based on the type of battery chemistry that you have) during each phase of the charging / maintenance cycle. Certainly seems as if it would result in the batteries not being fully charged and potentially shortening their life cycle.

Yeah thats exactly what I was thinking would happen - wasn't sure how much v-drop a diode causes. Man, bummer. Well I can just switch over the battery switches easy enough to keep all 3 batteries charged during long times of sitting at home - probably the better way to go anyway. Thank you for the info!

 

[Radio Guy]

If you have a dual battery system don’t you already have a diode isolating charging thingee between the alternator and batteries? If so you just connect your solar charge controller to the alternator output and let the rest of the system do its job.

For battery isolation I would use Schottky diodes with very low forward drop.

 

[Silverator]

Waaaaay back in the 70's my dad (his birthday today ironically) created devices that separated battery banks just as needed - he made them primarily for motor homes, campers, camper/boats, etc.. It was called a Divi-Charge, and yes - Diode based. Not sure what ever happened to the company, but you can still find them at times on eBay.

https://www.ebay.com/itm/1952838745...EUz8wXxGY8WIz6I3+TLjFu1TVa|tkp:Bk9SR6b-97OPZA

https://www.ebay.com/itm/166552699706?itmmeta=01J1XKNVT7T86H9EAVDVDKVGD6&hash=item26c750833a:g:gzMAAOSwzuBlpiRK&itmprp=enc:AQAJAAAAwLqkp9e3GC1jnkb9ccAcfnYlGuVOx1ShHqck1zOyKtWDmrHYUxc9xDAHJobICenPwSN7eo9gzABpiUq7a4kComGcjHL0XlDWzM6Qwj76SPB4yOM+VybbzYeDxs9zwMhvMK8CYiUceQQ1u5NaoW55Nbuc7uFK2xYsuLG9/9n6hfP5DdujhYqOJHVZaSJnKtpIo1ptHRdEtGKMA+90E+JrIk+uBhNrAHHKWjpSdlwn3nLgooIfsyT4nOejt4ZfLQRg+g==|tkp:Bk9SR5i917OPZA

Dayum. Just looking at those.....memories. There's good chance I de-burred holes on that one, or cut the extrusions or put on the stickers. It was a family business, and it makes me happy to see them still being used. He built that business off of sweat and faith right in the middle of the 70's gas crisis when NOBODY was buying rec vehicles an motorhomes. Still made a go of it. Happy Birthday Dad!!

 

[chorky]

If you have a dual battery system don’t you already have a diode isolating charging thingee between the alternator and batteries? If so you just connect your solar charge controller to the alternator output and let the rest of the system do its job.

No no diode. I will try to explain (hand drawings are terrible and I'm waiting for a contractor to draft up a full wiring diagram for me)

• From the starter battery
  • positive lead to Genesis smart solenoid
  • solenoid output through 150a fuse to 4ga cable
  • 4ga cable runs to bed of truck and connects to PRIMARY battery switch input #1
• House battery 1
  • 125a fuse to 4ga cable to SECONDARY battery switch input #1
• house battery 2
  • 125a fuse to 4ga cable to SECONDARY battery switch input #2
• SECONDARY battery switch
  • controls if battery 1, 2 or both are used to output
  • output to 4ga cable to INPUT of PRIMARY battery switch #2
• PRIMARY battery switch
  • controls if battery bank 1 (starter battery) or battery bank 2 (aux 1, aux 2 or both) provide power to 'house' system (lights, fridge, inverter, etc...)

The 'standard' position for switches

• PRIMARY battery switch - position 2
• SECONDARY battery switch - position 1+2

I suppose yes instead of having the output of the solar controller going strictly to the aux batteries, I could have it go to the connection point where the 4ga cable from the starter battery ties into input 1 of the primary battery switch. But then I would still have to have both switches on position 1+2 for the house batteries to then receive any sort of charge. The solar is primarily to keep the house batteries charged, I just was thinking maybe there was a way to provide a small trickle charge also to the starter battery for long periods of not driving if I did not want to or could not plug into shore power. I thin the easier method in this case is to just simply switch the primary battery switch from position 2 to 1+2 - so then the starter battery would in fact get charge from the solar system.

Hope that all made sense.

 

[Radio Guy]

Typically you have your stock battery and electrical system intact and add a secondary battery for stuff that you don’t want to run down the stock system with starter battery. You can even transfer the stock stereo system and other things to the new aux battery to keep them away from the starter battery. Then you charge both batteries via a battery isolator right off the alternator. You can abuse and kill your aux battery if you wish and will always have a good starter battery. No switches needed.

In the 70s I used to run dual batteries with a high current switch and the switch failed stranding me until I could get tools to rip it out and jumper everything. I never went back to a switch and always used a diode isolating charge thingee.

 

[chorky]

Typically you have your stock battery and electrical system intact and add a secondary battery for stuff that you don’t want to run down the stock system with starter battery. You can even transfer the stock stereo system and other things to the new aux battery to keep them away from the starter battery. Then you charge both batteries via a battery isolator right off the alternator. You can abuse and kill your aux battery if you wish and will always have a good starter battery. No switches needed.

In the 70s I used to run dual batteries with a high current switch and the switch failed stranding me until I could get tools to rip it out and jumper everything. I never went back to a switch and always used a diode isolating charge thingee.

Thats pretty much what I already have going on. But I was thinking of a way to provide some of the charge from the solar to the starting battery without having to hook the starting and aux batteries together for the very purpose of not draining down the starting battery.

 

[Radio Guy]

Thats pretty much what I already have going on. But I was thinking of a way to provide some of the charge from the solar to the starting battery without having to hook the starting and aux batteries together for the very purpose of not draining down the starting battery.

You shouldn’t have to connect the batteries together, they each serve a specific function and should be separated except for charging and the diode isolating charging thing does that. BTW, in my experience mechanical things like battery switches will fail before non mechanical electrical items.

 

[Mr._Bill]

Yeah I did think about that. And the charge controller is under $80, surprisingly cheap. But it's not waterproof so putting it under the hood is a no go. I could put it in my bed along with all the other electrical stuff, but I would have to rearrange it - for the 5th time.

But the bigger concern I would have is what that would do in regards to the solar panels. Can you have a ban of solar panels successfully running two different charge controllers? I have never seen anything hooked up like that before. But it's worth digging into.




Yeah thats exactly what I was thinking would happen - wasn't sure how much v-drop a diode causes. Man, bummer. Well I can just switch over the battery switches easy enough to keep all 3 batteries charged during long times of sitting at home - probably the better way to go anyway. Thank you for the info!

The solar panels should not have a problem feeding two charge controllers. Put the second one in the back, with everything else, and use the hot wire in the Tow wiring to charge the Main battery.

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