What makes you so certain?
Suppose the battery voltage dropped because someone switched off the charger for a few days. Wouldn't the charger be in the same situation as when the battery was loaded by the motor?

Don't see why.
A battery not charged for a few days looks like a lot different than the load of a starter motor.

The TS said the charger only failed when connected during the start process.
The relay disconnects the charger during that process to eliminate that as a source of failure.

 

Ive seen some worn out starters on small diesel engines draw as much as 400 amp and thats the starter running, the inrush current can sometimes peak at 500+ amps, As I said Ive seen these old large wheeled starter / chargers shit out in a puff of smoke when they been left connected also seen some very very expensive BSU's pop all the mosfets when overloaded.

Yes I know the best thing to do is switch off the charger but we cant fix stupid

 

Yes I know the best thing to do is switch off the charger but we cant fix stupid

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Few people actually read the instructions, and then follow them.
That makes me smarter than "your average Joe".

Few people want to pay what professionals expect.
That's why I quit working on Cars and Trucks ~35-years ago, its a cut-throat business.
I will only allow a Hot-Rod-Shop to touch my Truck,
at ~150.oo an hour,
if I don't have the appropriate equipment to do the needed work.
They're the only honest and intelligent Technicians I can find.

A worn-out Battery, or a loose Battery-Connection
can increase the amount of Current drawn by a Starter-Motor,
causing a "Booster/Charger" or a "BSU" to be over-stressed.

Anything that can cause a Starter-Motor to "stall" will increase it's Peak-Current-Draw.

A worn-out Starter-Motor may be in a such a state that will cause it to draw excessive-Current.

A Diesel-Engine, or any modern Gasoline-Engine, should start within roughly ~1-Second, or less,
if it doesn't, your just begging for trouble further down the road, ( or even immediately ).
Find the cause and repair it properly,
meaning,
replace everything that does not meet the manufacturers specifications, or better, or you'll pay later.

There should seldom be a circumstance where a "Booster/Charger" is "required" for any purpose,
at least since the year ~2000.
And, a better solution can always be found.

An Automotive or Truck-Shop should have a Maintenance-Charger for every "technician", ( mostly Clowns ),
and, appropriate Extension-Cords available,
and trained-technicians who are paid to use them correctly.

"Sometimes" You get what You pay for.

The best replacement for a "Booster/Charger is one or two NEW-Batteries, and a Maintenance-Charger, on a roll-around Cart.
There's nothing to break.
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A Diesel-Engine, or any modern Gasoline-Engine,

None of this set-up is modern, far from it

 

Even my loaner battery charger can survive a short circuit for a few seconds.
What I see is that depending on where the charger is connected, the voltage drop during cranking may be excessive. If the starter draw is 100 amps, then each milli-ohm of resistance is a tenth of a volt drop. And a starter will spin enough to start an engine with a lot less than 12 volts. So a check for any voltage drop in the battery to starter connections is what I recommend.

 

Where ever this establishment is .......... I'm staying far away.
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So now we have more questions: Just where is the charger connected?? Is it directly to the battery terminals??Or at the terminal at the solenoid?? And the engine block?? What is the current rating of the charger?? In fact, what brand and model is the charger?? I might have one. I do have a small cheap "electronic" battery charger that has no output but a nice set of leads.

 

The charger is connected direct to the battery,,,,,,,,,, current rating, brand and model, no idea, its housed in a metal housing that looks like its been painted about 20 times and they have about 7 of these chargers

 

Big transformer, old selenium rectifiers that can't take much abuse, very little smoothing and bugger all regulation... That's my guess...

 

And foolish parts-changers who will plug-in the Truck because it won't start,
and then immediately hold the Key in the "Start-Position",
while the Starter goes ............. Ahhrerrnn .............. Ahhrerrnn ............. Ahhrerrnn ............. Ahhrerrnn.
While Blue-Smoke starts slowly rolling out of the Starter.

Starters are designed to operate on good, Charged, Batteries, not a Booster/Charger, either built-in or external.

=================================================================

"" Problem is these people have no spare money so unfortunately the simplest solutions wont work ""

They have enough Cash to afford a Whole-House-Generator.

I certainly can't afford even the routine-maintenance on a Whole-House-Generator.

But I can easily afford a ~$50.oo Maintenance-Charger, on a fixed-Social-Security-Income.
( but a ~$50.oo Charger, plus a New, Premium, ~$250.oo Battery and Cables might require some planning )

~$50.oo is just one Oil and Filter(s) Change on a small Diesel-Engine, which is at least once a year.

The Generator very likely needs a New-Charger, and a New-Battery and Cables,
which will be good for another ~6-to ~7-years before these good-folks "cry-broke" again.

They may well be very nice people,
but what are You going to do when their Air-Conditioning-Unit, or Water-Heater dies ???

The only way this might work-out is if they are your Parents, or Grand-Parents.

The Money from the sale of the Generator could probably be put to better use.
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I have upgraded several of those older chargers by replacing the cheap selenium rectifiers, which do tend to fail, with adequately rated and adequately heat-sinked silicon rectifiers. By means of paying attention, the non-regulated charger does a good job. AND for instances of a rather dead battery, setting the switch to the six-volt position for a while works very well to limit the current.
AND these chargers are not for trucks, but for generator engines. So as for damaging the starter motors, the fact remains: "You Can't Fix STupid!!!" That is not likely to change, given the considerable effort to thwart natural selection.

 

So it would seem your choice is to get an upgraded/better charger or add the relay.

 

Actually there is an option with a "relay". Think about the older automotive voltage regulators that were used with a generator. There was a relay with a heavy wire coil and when the generator current got too high it would open the field circuit. I have seen the same scheme with a relay opening the circuit like a buzzer. THAT could be simple scheme to protect the chargers from excess power dissipation during any over-current condition. It would require rewinding a relay with a few turns of much heavier wire..

 

There seems to be around ~3 concurrent stories going on in this Thread,
all of which seem to be having to do with putting a "Band-Aid" on an expensive piece of equipment.
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Actually there is an option with a "relay". Think about the older automotive voltage regulators that were used with a generator. There was a relay with a heavy wire coil and when the generator current got too high it would open the field circuit. I have seen the same scheme with a relay opening the circuit like a buzzer. THAT could be simple scheme to protect the chargers from excess power dissipation during any over-current condition. It would require rewinding a relay with a few turns of much heavier wire..

Sounds good, tell me more

 

Are the glow-plugs working?

 

Sounds good, tell me more

OK, the circuit for the current limiting relay is identical to the circuit for a simple electrical buzzer, where the current is interrupted when the magnetic attraction causes the contacts to open. In the older three-relay voltage regulators that contact-opening relay also had another coil that I am not sure what it was supposed to do. (By the time I started driving most cars were all 12 volts with alternators, so I never had to learn exactly how the three-relay regulators worked.) But I had already had discarded ones to take apart and look at.
Probably adding some turns of heavy wire around the coil of an existing relay could provide the required contact opening.
If you know the rated current of those chargers you could experiment and make a relay coil that would operate when the load approached that amount..

 

A Second thought!!
Since the chargers are connected constantly, the normal current should be quite low, probably less than one amp. So the similar but much simpler scheme will be to simply put a 12 volt light, such as a "stop-light" in series with the charging connection. At the low current the filament will have a low resistance and so the battery can charge enough to be close to fully charged, but when the engine cranking load drops the battery voltage, the bulb will light, and the resistance will increase, and the current will be limited to whatever the bulbs rated current is. THAT should protect the charger from an overload.