how low can 24v battery drop and still start? I have dropped mine down to 23.9 and it starts. They are 6 month old batteries. How low can it go and still start. My inverter shuts off at 20 volts. What is a safe voltage that will start the truck still. your advice will be much appreciated.
About 3 feet, and more of a drop and it will smash 
Unless theres been some major changes since i left the spanners working on trucks instead of drivin em, as far as im aware, there not 24volt batterys, there normally 2 12 volt wired in series. To answer the question, not to much of a drop, 10 volts maybe. so obviously double that.
ukjon1:
how low can 24v battery drop and still start? I have dropped mine down to 23.9 and it starts. They are 6 month old batteries. How low can it go and still start. My inverter shuts off at 20 volts. What is a safe voltage that will start the truck still. your advice will be much appreciated.
22 Volt
If lower you need some luck/Normally Roomheater ect,ect get switched off by a Relais by 22 Volt.When Engine runs gets charging Voltage up to 28 Volt
I would say about 22 volts too,I was down to 21.8 on my Merc a few years ago and it would’nt start.
A single battery in ideal condition works at 13.8 volts with the engine running, a knackered one will be around 9.7 volts or less
My Waeco fridge used to shut down at 20.8 volts and the truck would still start after the weekend. I would say 19.8 v as a minimum
You need volts but a starter motor uses amps
Noworries:
About 3 feet, and more of a drop and it will smash
Drop testing is not good for batteries 
Even when you do it properly
Thanks for your help. i might try dropping to 23volts. Save me charging my batteries every 1hour. Will dropping them 3 feet help?
The outside temp will also play a part in how much power is needed to crank the engine, the colder the outside temp the quicker the engine will cool and the colder it becomes the more juice it will need to start, as the oil will become thicker also, some batteries will be better than others as they will have more cranking amps available ,
Best to carry a decent set of jump leads with you, or if you are feeling flush a portable 24v battery pack that you can keep fully charged from the vehicles aux or cigar lighter whilst driving bearing in mind a good one can set you back several hundred pounds if it bears the name snap-on
It s not about the volts, but the amperage left in them I have seen a fully charged 12 volt battery ( your all right BTW trucks work on 2x 12 to give the 24) alongside a dead battery start a truck 24 volt is only what it works on, if it cant produce the needed amperage it doesnt matter wether its 9, 12 or 24 volt
ukjon1:
how low can 24v battery drop and still start? I have dropped mine down to 23.9 and it starts. They are 6 month old batteries. How low can it go and still start. My inverter shuts off at 20 volts. What is a safe voltage that will start the truck still. your advice will be much appreciated.
Just started mine on 21.7 bit worried for the morning I had one battery replaced a couple of months ago maybe they should have done both?
To be honest your starter motor is more concerned about the ENERGY your battery has, the voltage is potential energy and is a bit like pressure, amps are the rate that electritity can flow across whatever the energy needs to work (ie the starter motor) the most important thing is how many WATTS your battery can produce (energy). A knackered battery even showing 24 volts (even knackered a battery can show 24 volts but have little energy) may be unable to produce the amount of amps to give your starter motor enough energy to turn over, W (energy) = V (volts) X I (amps).
So if your battery is knackered it may show 24 volts across its terminals but even if you short circuited it may not be able to provide the amps which would work together to make the watts (the energy) to get the starter to turn (remember the starter need watts W which equal volts V times power I.
However a really healthy battery, may, even if it is a bit low on voltage (bit undercharged may be able to supply enough amps to give enough energy (volts x amps) to turn over the starter.
If you don’t believe me you could always do physics and check for yourself 
P.S. a battery is a series of cells so 2 X 12 volt batteries connected in series is a 24V battery since this is only an extension of the number of cells connected in series.
schrodingers cat:
P.S. a battery is a series of cells so 2 X 12 volt batteries connected in series is a 24V battery since this is only an extension of the number of cells connected in series.
I remember cars with 2 x 6 volt batteries on them wired the same way. Each cell is worth 2 volts
Wheel Nut:
Noworries:
About 3 feet, and more of a drop and it will smashDrop testing is not good for batteries
Even when you do it properly
Fair play to both of ya! Made me giggle that did!
As mentioned above its more to do wtih the stored energy in the battery, so the point at whitch it can no longer start the truck depends on allot more factors then just the no load voltage on the batteries, the voltage determines the speed that the starter motor will spin at however when the load of the motor is put accross the batteries the voltage will drop on a healty battery it will drop one or two voltes however on a failing battery the voltage will drop significantly redunceing the amps avalible, some very high capacity batteris that are down on voltage could start the engine where as lower capacity batteries even with a higher voltage wont also there is a hugh diffrence in the power it takes to start engines, some start almost streight away some take a bit more spinning
I concur with Rikki-Uk.
It’s not the volts, its the amps.
If it were the volts, then you could string 16 torch batteries together, and expect to turn over an engine!
From experience, I’d say you need at least 84 cranking amps to turn over a 2litre car engine.
I believe car batteries are around 108amps when fully charged, so a drop to 84 amps represents only a 20% fall compared to full power - at which point the starter will be falling over!
I’m not sure if you can directly proportion it up to make a factor that works on tractor units, but if the 24v supply is just two car batteries in series, then presumably we’re looking at 216amps to start, dropping to 168 before the starter won’t crank…
Try putting a meter across a clock battery (1.5v) when the clock starts losing time (ie you know it’s now flat!) and you’ll see it still measures something like 1.2v rather than the anticipated <0.1v that you might otherwise expect from a flat 1.5v battery. Once again, you’re looking at a 20% drop off, but this time you’re seeing it in the volt output, rather than the amps which are negligable in a small battery. 
Volts, in the terms people are talking about here are irrelevant. A dead battery could still display close to 24v. A volt is a measurement of electrical potential.
The only way to test a battery is to load test it, not something you can measure with a voltmeter across the terminals.
I still don’t have a great grasp of electricity, batteries are lot of black magic that somehow turn a chemical reaction into electrical energy, but it’s the formulas that are important. The relationships with power (watts), current (amps) and voltage.
Batteries are measured in ampere hours, so just saying amps, is misleading. 100ah battery could in theory supply a current of 1 amp for 100 hours, or combinations thereof (100 amps, for 1 hour for example).
Cold cranking amps, however, is an important measurement, healthy batteries should be able to supply a percentage of their cold cranking amps (I forget the percentage) for a sustained period of time (I forget the time) without dropping the voltage too much.
Just remember that with great power, comes great current squared times resistance
A key parameter of a battery is the internal resistance, which acts as a limit to the current you can draw at a particular voltage: I(max) = V / R(internal). As a battery ages, the internal resistance tends to rise, leading to the linked phenomena of an apparently charged battery that can’t spin the starter fast enough to start the engine and/or an apparently charged battery that dies when the glow plugs kick in on a cold start.
The type of batteries typically used on vehicles are damaged by deep discharge, which is why many 24V fridges and similar switch off as the voltage drops. There are lead acid batteries specially designed for deep discharge that are used for applications such as golf buggies and electric wheelchairs.
The wealth of technical knowledge shown on the scientifics of batteries here just goes to prove the worth of Google and the capability of being able to copy and paste.
so the morale is,
dont run the batteries down or your [zb]ed.