New ambulance build - electrical system design

[sstrang]

Hi all -

New to the forum and to ambulance conversions but am excited to be here!

A little back story - I picked up a '94 4x4 Chevy ambulance (wheeled coach) a few weeks ago and built out the interior for a 2 week camping trip in the SW. I didn't have time to set up electrical / diesel heater prior to the trip, and now that we're back want to get this done ASAP for more winter camping. It's been slow to start on cold mornings, and as I plan to use off-grid for winter camping / skiing trips I want to make sure I'm completely covered for cold starts (currently has two new batteries, 950cca each). I'm looking to do something like this, but want to check on a few things...

*Note: I kept this high-level and am aware it's missing things like fuses, etc.

The idea is to set this up so that 1) in cold weather I can start the car with both auxiliary and starter batteries, and 2) I can charge starter batteries from shore if necessary. I think theoretically this should be possible, however I think it comes down to getting the right charger, and potentially the rating of the alternator. Also, I'm totally new to this so might be missing something entirely.

Has anyone done / seen anything like this before, and is there anything else I should consider? Any help would be greatly appreciated!

Some pics of ambulance / trip below:

 

[iggi]

Everything you want can be accomplished with this: https://www.bluesea.com/products/76...rging_Relay_with_Manual_Control_-_12V_DC_500A

Unless you're running lithium for your auxiliary battery/ies, in which case many people recommend going with a DC to DC charger to avoid overheating your alternator. You'd also then likely want to use an echo or relay charger from the Aux battery/ies to the starting batteries.

 

[sstrang]

Thanks, think this looks perfect! Planning on AGMs so won't have lithium for now. If I understand correctly this sits directly between the two battery banks. In the automatic position, it naturally sits with the two banks disconnected. If one battery starts charging, due to either the alternator or the shore power, it will begin to charge both sets of batteries. Additionally, for the cold start situation, I can manually force it to connect the two so that both sets of batteries provide power for the starter.

Two quick questions. Is there any risk of overcharging the starter batteries from shore power? Not exactly positive how 120-12V chargers work and wonder if they would only detect the power in the directly connected aux batteries and not the starter ones.

Also, should I be concerned with the distance between the two sets of batteries? Right now the starter batteries are under the chassis behind the passenger seat and I'm planning on putting aux batteries in compartment by the driver rear wheels.

 

[iggi]

Thanks, think this looks perfect! Planning on AGMs so won't have lithium for now. If I understand correctly this sits directly between the two battery banks. In the automatic position, it naturally sits with the two banks disconnected. If one battery starts charging, due to either the alternator or the shore power, it will begin to charge both sets of batteries. Additionally, for the cold start situation, I can manually force it to connect the two so that both sets of batteries provide power for the starter.

As I understand Blue Sea's documentation, yes.

Two quick questions. Is there any risk of overcharging the starter batteries from shore power? Not exactly positive how 120-12V chargers work and wonder if they would only detect the power in the directly connected aux batteries and not the starter ones.

No.

Also, should I be concerned with the distance between the two sets of batteries? Right now the starter batteries are under the chassis behind the passenger seat and I'm planning on putting aux batteries in compartment by the driver rear wheels.

I don't believe it's a big factor if you use high quality and properly sized cables but a qualified engineer ( @luthj ) might have better advice.

 

[luthj]

I don't believe it's a big factor if you use high quality and properly sized cables but a qualified engineer ( @luthj ) might have better advice.

As long as you size the cables properly, you can get good charge rates between the two. Depending on alternator voltage, you may want to aim for less than 0.15V drop at somewhere around 20A. Ideally you could get 50A with that voltage drop, but its not required if you get most of your charging from shore power or solar. Remember to account for total round trip cable distance, not just one leg.

Obviously you also need to size for your max current. For example boosting the starter battery would mean fusing for 200A and using 0 gauge or larger.

Do not use diode based isolators. They have a 0.2-0.9V internal drop, which wrecks any chances of good charging across the isolator. Voltage sensing relays work fine.

If you don't need the voltage logic, a simple dumb relay wired to your alternators D+ signal will connect the batteries when the engine is running. You can also use a simple switch for manual combine if needed.

 

[sstrang]

Thanks for all the input - main takeaway being that isolator or charging relays sit between batteries, not between charging source and batteries (something like this).


Verkstad, thanks for pointing out the shutoff between aux and inverter. Do most people just use a manual switch or is there a way to wire in something to detect if shore power is present?

Lastly, it might sound kind of silly, but tactically speaking what is the best way to approach of separating the starter batteries from the aux systems and connecting just to the cab / truck? I know some people do this at the seat disconnect switch, however the switch in this truck actually cuts power to all systems (including cab and alternator) so I can't do it there. I'd like to avoid fully disassembling the interior and dash if possible but know it might come to that. I've got lots of pictures of what I've been able to get to so far and can share if helpful.

Thanks again for all the help

 

[iggi]

Could save yourself some trouble by purchasing a combo inverter/charger. 120V goes direct to it and the AC passthrough is included.

>>Lastly, it might sound kind of silly, but tactically speaking what is the best way to approach of separating the starter batteries from the aux systems and connecting just to the cab / truck?

Just as you've written it. Source all the cab power, lights, etc from the starter batteries. And keep the Aux isolated to the aux batteries. A bit of time with a multimeter can save you a lot time taking things apart.

 

[Joe917]

Is your diesel heater a coolant heater? If so plumb it into the engine coolant loop. You can then heat the back from the engine when on the move and pre heat the engine for cold starts.