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 Power units/Elektroblocks 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 poor and flat that it places an overload on the Alternator, potentially burning it out. 

Imagine a worst case battery where each cell is shorted out? That might be pretty much the same as putting an iron bar across the alternator terminals.

Most Starter batteries that are not up to starting the engine are likely to be already suffering failure, not just needing a charge. A battery already suffering issues and dead flat will be a load few charging systems will like. 

Even if the Alternator survives the first challenge, the battery will be so much lower than it is designed to drop to that it may draw a big charge from the Alternator which may be applied for some time, overheating the battery, and/or alternator, causing additional damage to an already ailing Starter battery. 

This additional damage to the battery can then overload the Alternator at some point in the future. 


So just because the Alternator doesn't fail straight away, doesn't mean it won't do so further down the line.


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.



There is also the risk of the Jump leads being connected incorrectly causing reverse polarity, shorts, etc.


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



We would always advise you never jump start a Motorhome Starter Battery that is flat. Even it is brand new, it will have suffered in running down so low, so just replace it.

If you have a Varta LFD 90 as the habitation battery, then consider installing the habitation LFD 'under the bonnet' as a temporary measure. Then maybe buy a new LFD 90 to replace the relocated old habitation battery, 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 also 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 (the exception might be a Tracker/Alarm drained Starter battery) so swapping them around will extend the Habitation batteries useful life, giving 6 - 8 years overall life for both of them in the average Motorhome.  



Jump Start power packs.

The 'Jumpstart Laptop Battery' packs are generally only sized for smaller Petrol engines, although sold as suitable for bigger Diesels some are marginal at best. One of these units that has just been charged will have a higher 'capacity' than one that has been in the glove box a week or more. Therefore, there are sometimes exaggerated claims based on 'Test' results where the battery pack has just come 'off charge' so it's true capability may be very different in real life. 



If you have, one try and charge it in someone else's vehicle just prior to use, even 15 minutes charge makes a big difference.


Again all the warnings re damage to the Habitation electrics and Alternator apply to these 'jump start' units as well. 

We would advise they are not used on a Motorhome and advise caution on a Car.


 

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 trouble is false economy. 

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