Jump Starting a Motorhome : Why you should think twice.



If the Starter Battery runs flat on the average Motorhome and you then jump start it, there is a risk of damaging the Motorhome Habitation Electrics, the Battery and Alternator.


When the Starter battery runs flat, but the habitation and batteries are still charged, an attempt to Jump Start the vehicle battery can result in power being drawn by the Starter Motor from the habitation battery bank.

In many cases this will be through the Sargent, BCA, Schaudt, etc. Charger/Power Controller and any associated components that will not be rated for the 200 amps the Starter Motor might try and draw.


We have seen several vehicles with damaged Electroblocks/Power units following a Jump Start.


The circumstances will depend on many factors, but what can happen is the engine starts turning but the power available from the jump start source is not enough to turn the engine over fast enough to start. The Alternator energises and operates the D+ circuitry to join the Habitation battery to the Starter Battery for charging. 

From this point on current can flow from the Habitation Batteries to the Starter Motor. 

Fuses may blow, but usually too late.  



In addition to the above the Starter battery may be so flat that it places an overload on the Alternator, burning it out.


Even if the Alternator survives, the battery will be so much lower than it is designed to drop to, that it will draw a huge charge from the Alternator which may be applied for some time, overheating the battery and causing significant additional damage to the Starter battery. It may damage the Alternator at this point, i.e some time into charging, as oppossed to soon after starting.


If the battery is tired AND flat, the risk of Alternator failure will be higher.


In the 'old days' when this happened to our cars, it was not uncommon to put 'appliances' on in the car, like Heated rear window, Blower Motor, etc to draw some charge away form the battery so it charged more slowly with a lower risk of damage,

However, a modern Motorhome Alternator is so powerful, even if you put everything on, there would still be enough to fry the battery.      


If you also throw into the mix that a 'poor starting' Motorhome may also have poor Earth straps, then the potential strain on the Alternator will be even greater.



A low Starter battery will draw all it wants form the Alternator, which can be much more than any other connected batteries, like the Habitation batteries, which may only take a low current.  


There is also the risk of the Jump leads being connected incorrectly causing reverse polarity, shorts, etc and one of the reasons why many Garages advise against having the ignition on in the donor van to reduce damage to the Donor. 


However if you can persuade the owner of the donor vehicle to connect the jump leads and risk blowing his Alternator to charge up your battery for 30 mins before attempting a Jump start, then it will reduce the risk of damaging the Habitation electrics, but it creates a whole list of other issues and potential costs.



We would always adise you never jump start a Motorhome Starter Battery that is flat. It WILL have suffered in running down so low, so just replace it.

If you have Varta LFD 90's as the habitation battery, then swap them over, they make brilliant Starter batteries and are the exact same Starter battery size for most Motorhomes after 2001.


Having Varta LFD's for both Starter and Habitation batteries is a great insurance policy and they can be swapped around after 3 years to extend the life of the habitation battery. 

A habitation battery works three times as hard as a Starter battery, so swapping them around will extend the Habitation batteries useful life, giving 6 - 8 years overall life for both in the average Motorhome.  


The Jumstart 'Laptop Battery' packs are generally only sized for smaller Petrol engines. There are exagerated claims based on 'Test' results where the battery has just come 'off charge' where it's true capability will be quite different to having sat  distorted. Hence 'Real World' 

If you have one try and charge it 


 

We suggest you investigate and rectify the cause of poor starting the minute it first occurs, hanging on to a Starter battery after it first shows signs of failure is false economy. 

But don't neglect 'poor' Earths straps as a possible cause of a 'slugish' Starter.