I think the Op has axed the idea of employing the 36v pump.
There are 36vDC power supplies, with adjustable voltage output. Not sure of their range, if they go as high as 44v for proper 12v lead acid battery charging.
Op could get 3 batteries in series to power the pump, and use a 12vdc inverter to power the 36v power supply to charge the batteries from alternator, then that night unplug from inverter and plug into the grid to top off the batteries.
I'm slanted towards power supplies as they can hold voltage high enough for long enough, whereas most smart chargers do not really care about reaching a true 100% state of charge, but deeply cycled batteries Do care, and will live much longer and perform better when they reach this ideal often
If using 3 smaller AGM batteries in series, with an adjustablevoltage power supply set to 44v, and an ammeter, one can see how long it takes for amperage to taper to 0.5% of the battery's capacity( 20 hr rate), then later on use a spring wound timer to seek and hold that voltage for a certain period of time, if the usage/depletion level is similar. As the batteries accumulate cycles, that time will have to be extended.
I've been using a Meanwell power supply rated at 500 watts since october 2014. It can output 40 amps(~600 watts) at any voltage from 13.12 to 19.23. Its specs state a lesser voltage range and lesser output wattage, but my tools are not lying about its range. it is also power factor corrected, meaning more efficient than a NON pfc power supply would be.
Meanwell has many many different power supplies available, but what is most important when using them as battery chargers is that they say 'constant current limiting on overload' in the spec sheet
Mine is the RSP series, specifically the rsp-500-15. and was 112$ delivered in 2014.
there is a rsp-320-36 320 watts and 36v.
MEAN WELL is one of the world's few standard power supply mainly professional manufacturers, covering 0.5 to 25,600W products are widely used in industrial control, medical and other fields, in line with international safety certification, short delivery of spot inventory.
www.meanwell.com
It says its range is only as high as 39.6v, which would only be adequate for 3 agms in series at float voltage. The actual upper voltaeg range is likely higher but one would have to buy one to know how much higher.
The rsp-320-48 stated voltage range is 41 to 56v.
Three 50% depleted AGMS in series should be brought to 43.2 to 44.1v for no less than 5.5 hours.
The rsp-320-48 is 51$ on mouser. it can likely output 400 watts.
If someone told me to make the 36v pump work, I'd likely get 3 UB12350 (35amp hour) agm batteries wired in series, and use the rsp-320-48 set to 44.1v, to charge them that night. If they did not have enough capacity to make it through the day I'd add a pure sine wave inverter to the truck's 12v battery and power the power supply through it to charge the 36v bank while the engine is running, and that night, unplug meanwell from inverter and plug it into the grid. I'd hold the 35 AH agms at 44.1v until amperage tapered to 0.17 amps, then disconnect batteries from charging source.
There is an RSP-500-48 version too, for 121$. This 500 watt version, would be better if larger batteries are required and more recharging is required while the engine is running but it would also require a larger inverter.
Not sure these really require a pure sine wave inverter though, they might work fine on cheaper Modified square wave inverters. I've never tried to power mine from an inverter and will not try unless there is a madmax type of scenario. I'd not fear powering it from a capable quality PSW inverter but only reluctantly do so from a MSW inverter, unless MEanWell outright said that MSW is fine.
