Jump starting dual battery setup (1 Viewer)

[kgrove]

I searched and surprisingly didn't find this topic - maybe I just used the wrong search terms...

What's the best way to jump a dual battery system 24v starter if you're limited to a couple charging packs?

I've got an HDJ81 at port that I'm trying to have picked up and brought to me. The nature of where it is, I think I'm limited to portable charging blocks to jump it as the trucker picking it up will be in a big ass car carrier and won't be able to maneuver close to the vehicle to connect jumper cables and the port doesn't have the facilities to allow me to connect it to an ac powered charger overnight. They tried once with two portable charging packs and couldn't get it to turn over, but I wasn't there to see how they did it. The port is particularly non-helpful and there is no help around to assist the driver in any way.

I'm assuming the best way under these limiting conditions would be to connect the two chargers separately to each battery, not both to one battery? Or if they are able to get a car near enough for jumper cables, attach both portable charging blocks to one battery and the jumper cables to the other battery? My thought process is that connecting to one battery is fine if the charger has the time and power to try and charge both batteries (like if you can connect to an ac powered charger or a running car with jumper cables), but if it's a one shot thing, you want to try and get both batteries over 12v, not one battery over 12 and one still stuck at 11.

 

[HillCountryTX]

Hey @kgrove, maybe this will help (see HDJ81 Stock Dual Battery Questions - https://forum.ih8mud.com/threads/hdj81-stock-dual-battery-questions.144807/post-13562746 ) To answer your question, yes, two separate chargers charging both batteries will make it go faster. However, if one of your batteries is ruined (dead cell), it will drag the other battery down with it, the achilles heel of this 12v/24v system setup. Electrically all you need is to connect to a single battery to charge both batteries. Remember, this factory 24v starting system is only operating at 24 volts when you are running the starter (i.e. actively cranking the engine)...any other time it behaves as a 12v system that just happens to have two batteries in parallel.

The issue is in the grounding of the passenger side battery (the one that has the protective cover and warning text about NOT using it to jump start!) when you go to crank the engine. There is a 12v/24v relay switch that puts the two batteries in series (thus the voltages add...12v + 12v = 24v) ONLY when you are cranking the starter....every other time it is a 12v system with two batteries. When the batteries are in parallel they have the same voltage, but their capacities (amp hours, or aH) are added together. If you place two different charging packs on the two separate batteries and connect the positive/negative charging pack leads to the corresponding battery terminals you are trying to charge, then it won't matter. The issue is if on the charging pack you are using to jump your passenger side battery (the battery with the warning cover on it) you connect your negative lead to the chassis rather than the negative terminal of the battery you risk having a full 24volts on your charging pack on the passenger side battery...not good.

 

[kgrove]

Hey @kgrove, maybe this will help (see HDJ81 Stock Dual Battery Questions - https://forum.ih8mud.com/threads/hdj81-stock-dual-battery-questions.144807/post-13562746 ) To answer your question, yes, two separate chargers charging both batteries will make it go faster. However, if one of your batteries is ruined (dead cell), it will drag the other battery down with it, the achilles heel of this 12v/24v system setup. Electrically all you need is to connect to a single battery to charge both batteries. Remember, this factory 24v starting system is only operating at 24 volts when you are running the starter (i.e. actively cranking the engine)...any other time it behaves as a 12v system that just happens to have two batteries in parallel.

The issue is in the grounding of the passenger side battery (the one that has the protective cover and warning text about NOT using it to jump start!) when you go to crank the engine. There is a 12v/24v relay switch that puts the two batteries in series (thus the voltages add...12v + 12v = 24v) ONLY when you are cranking the starter....every other time it is a 12v system with two batteries. When the batteries are in parallel they have the same voltage, but their capacities (amp hours, or aH) are added together. If you place two different charging packs on the two separate batteries and connect the positive/negative charging pack leads to the corresponding battery terminals you are trying to charge, then it won't matter. The issue is if on the charging pack you are using to jump your passenger side battery (the battery with the warning cover on it) you connect your negative lead to the chassis rather than the negative terminal of the battery you risk having a full 24volts on your charging pack on the passenger side battery...not good.

Great info, and thanks for the reply. I'll study that link and make sure I can translate that to the trucker picking up the car.

I've thought long and hard about converting the starter to 12v to simplify things long term, but that won't help this one sitting on the customs dock in it's original form. I live in Phoenix, so cold weather starts are about as far from my biggest concerns as you can get! A cold start to me is if my garage is below 60F.

 

[HillCountryTX]

Hope it goes well. Please know, I am not an expert at the HDJ81 starting/charging system by any means , so please be cautious. However, since I have been through a dead battery(ies) on our HDJ81 I figured I'd share what I learned from the experience.

I too have considered doing the 12v starter conversion; and there are several threads on the conversion on 'Mud that should help. I am in NW Arkansas and even though we can get pretty cold (minus 20 deg. F back in Feb. of 2021), I'm guessing that the 12v conversion would be able to handle the cold(er) starts, as opposed to absolutely needing the 24v starter setup. From what I've read, please know that in addition to a new 12v starter (since the factory one is 24v), you will need additional, larger wiring in various places as the current is much higher in the 12v world than the 24v world. Having a dual battery setup with an isolator would be ideal, especially if you have a winch, or if you'd like to use your second battery to self-jump your primary starting battery...that is worth quite a bit in my book.