Crank battery drain

It sounds like there is a parasitic drain somewhere. Have you used a multimetre to test what is being drained from the battery when stationary?

The problem with ambulances and emergency vehicles in general is the amount of accessory wiring going to all manner of stuff. You would need to start running down individual connections and identify splices into the wiring looms.

A couple of thoughts. Is the vehicle alarmed and did it have a positioning system fitted in particular. Those use the mobile network so if the phone subscription was discontinued at the time of sale but the unit not removed or disconnected, the unit will still be polling for available networks and causes a big drain.

Assuming the cranker is 100Ah that works out to around 200mA draw on the battery over 3 weeks to completely flatten it.

Depending on what is connected to the battery there might not be anything wrong, it is not uncommon with many new vehicles to flatten the battery in that period of time due to standby current drawn by the many electronic devices installed in the vehicle.

Might also be a good idea to look for any un-standed devices that may have been left in the Ambulance like trackers etc.

18
Exactly what is the definition of FLAT? Is it flat, ie practically nothing or not quite enough to crank AND hold sufficient voltage while doing so to enable the ECU to provide injector pulses?

I have a DC DC unit and it sucks a small standby current from the main battery to stay awake ready for the main to begin increasing and then using charge voltage/current. I presume yours does as well.
The ECU and other standard items draw a small ma current too. Then there is the items Mick suggested may be still there in the background. So after 3 weeks what is the state of the battery in real terms. Most vehicles will be a bit slow or very reluctant after 3 weeks of standing. Many people return home after a few weeks away and find their town car battery has depleted to no crank stage and then gone further into discharge and stuffed the battery. Personally I would NEVER leave a battery on a circuit without some form of maintenance charging system attached if leaving for that period or longer. Do you really have a battery discharge problem? It should not be difficult to provide a small solar reg to assist the crank battery OR when on power a plug pack which dispenses 12v or a bit more and a few milliamps to keep the crank battery in top order.

Best option for the modern vehicle that can be parked up for weeks at a time is to fit a battery switch.
Looking at my Ranger's workshop book they suggest disconnecting the battery if parked up for 3 weeks or more.
So it's not something new that the vehicle itself is a parasite draw on the battery.
Of course you could fit a small solar that connects independently to the vehicle battery similar to this
that would keep the prime battery charged.
https://www.ebay.com.au/itm/20W-12V-MONO-SOLAR-PANEL-REGULATOR-TRICKLE-BATTERY-CHARGER-RV-AGM-Complete-Kit/351927999144?hash=item51f08b1aa8:g:p8IAAOSw2zNcueW7

Or a 1/ 2 step controller so when the camper battery is charged it would divert the charge to the vehicle battery.
Similar to this
https://www.facebook.com/groups/mandurahbuyandsell/permalink/4034322306579700/?sale_post_id=4034322306579700.

I will admit I'm selling this one.
I had it on my Ranger when I was away camping and not starting the ute for a few weeks at a time.
All power was fed in by 160W solar on the canopy and 2x120Ah batteries just running an Engel and 3 LED lights, so excess power was fed into the Ranger's battery.

Ron

You can try to trace the parasitic drain by measuring the current draw and isolating individual circuits. Start by removing all fuses, then re-inserting them one at a time. Remember that there may be a time delay on some circuits eg cabin lights which may stay in for a few minutes. Once you identify the circuit with the current draw, you then have to assess whether it is a legitimate current draw, or a leakage. That is when the fun begins...

How are the chargers hooked up?

Is the DCDC wired into the ignition circuit, or just links the batteries when the voltage gets above a preset voltage?

You might consider hooking the solar and Ctek up to the starter battery so it keeps it at peak charge and then the DCDC can do it's thing keeping the house batteries charged. If input is cut off for any reason, then the house batteries may get run down, but the starter should still be good.
Of just have an extra Anderson plug hooked up to the starter battery and plug the solar panel or the Ctek in for a couple of hours each week.

I have a newish BT50 and the parasitic drain on that will see me not being able to start it after 2-3 weeks, even with a brand new battery and no one opening doors or anything. Only accessories are a dash cam and a UHF that's turned off. ECU's and the clock always need a tiny bit of power and are enough to drain a battery if left long enough.

Hi 18,

You have not said if your “house” lights are connected to your “house batteries”. If they are not, then they will be running off your cranking battery. If you are using your interior lights for any length of time each night, then that may be the cause of your battery drain.

Macca.

I have similar and have noticed that the aux battery and main will drop together, today the aux was 12.3 and the main was 12.3, dc-dc was on float, but supposedly isnt charging till it gets voltage from accessories circuit of main system. Lights are always on whilst batteries are connected to the charger. This has never happened previous to my car getting 3 months to a tank of fuel.

.
Hi 18,

When vehicles such as the Sprinter are converted to ambulances, extra lights are added and for convenience some of these are simply connected to the original wiring loom. Hence they are supplied from the original cranking battery. This is not a problem for an operational ambulance as they do not spend long periods with the engine shut down, however if camped for a number of days and using those lights, the cranking battery will become discharged.

An easy way to check for this is to disconnect the main negative connection from the crank battery then try using lights. Any that now do NOT work must be have been supplied from the crank battery and should not be used with the motor off for lengthy periods. It may only be one or two lights but that would be enough to flatten the battery.

To overcome the problem, any such lights should not be used for camping or should be re-wired to the 'house' battery.

All Modern Vehicles with ECU will flatten the Starter Bat within 3 weeks so 2 thing can be done put on a slow 2 amp charger or fit a bat switch this also goes for your Caravan or camper trailer bateries