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Electric’s ? Simple?
A lot of people dabble with electronics for their car, boat or around the house, but even professional workshops and contractors
have to look for help as the market for specialist areas is vast. Something goes wrong under the bonnet of my car and I would
take it to a garage were the mechanics are trained specifically for engine management, but I do find even mechanics don’t
understand fully about charging circuits and battery chemistry. I’ve been working in power/charging systems for years
and can go on for ages about something as simple as a battery connection but I will try to make examples as simple as possible
to highlight principles for what to look out for and why you may need me!
a) If you consider; you have a 12V battery with a 12V charger, if you had 4 mtrs of 1mm cable between the two you would
get voltage drop, this is where some power is lost along the cable. If we say you loose half a volt it then means the battery
is only seeing 11.5V from the charger which means your battery will never be fully charged up wasting power and reducing the
life span of its internal cells.
b) D+ control from alternators are a common charging system used that I often have to replace, they work by taking a signal
from the alternator when its running to close a relay contact and put charge to a second battery somewhere on the vehicle
/ caravan / trailer. Often this system consists of a 30A relay and some 2mm cable run through the vehicle. The problem with
this is it has no intelligence same as simple junction split chargers. If your cranking battery is flat and your second battery
is 90% full the alternator will start by putting a bulk charge into your cranking which will obviously also go into your second
battery as well which can cause gassing. Or you may get it the other way round that it is only topping up your batteries as
the second battery is showing higher voltage causing your cranking battery not to see a full correct charge.
c) Traction vs cranking batteries are a favourite argument. Traction / leisure batteries are designed to give a continues
amount of power to run lights, electric motors, etc, but cranking batteries are designed to start an engine by putting out
a huge amount of power quickly. "These are the same batteries with different labels though?" WRONG! This idea came about because
a few manufactures re-badged surplus stock of cranking batteries as leisure batteries for the caravan and boat markets in
the 1980’s. The internal design is very different, the area and thickness of the lead plates inside are very different.
For second batteries on vehicles and boats I would always recommend GEL or AGM batteries due to the fact the charge and discharge
characteristics are ideal and they can recover from being completely flattened. Wet cranking batteries reduce lifespan considerably
when flattened completely. I wouldn’t recommend a GEL or AGM to be used for regular cranking only though.
The downfall with GEL batteries is the cost, you may pay £40 for a 100Ahr wet cranking battery and a 100Ahr GEL would be
£150.
Ambulances ‘eat’ second batteries due to the amount of equipment onboard, some workshops would not pay for
GEL batteries as they worked on the fact you could buy 3 wet batteries for the cost of 1 GEL. This is true but a GEL will
normally last 10 times as long as a wet battery and how much does it cost to take the vehicle out of service and replace the
battery ?
d) "My battery reads 12.8V with nothing connected, but goes flat in half the time it used to". This is another common problem,
even garages using a proper Drop tester can get caught out with this. The best analogy is to think of a battery like a lake;
the surface area is the voltage and the depth of the lake is its Ahr. As time goes by the lake may look the same area across
the top, but it may be silting up at the bottom to the extent that it is half the depth it used to be, but you cannot tell
from the surface.
Most people will check the voltage and think the battery is OK if its above 12V. Garages will use their drop tester for
a few seconds which will show the battery has enough power to start an engine, but the only way to measure a battery accurately
is to measure how much power you can take out and how much you put back in.
Three most common requests;
1) Fixing existing auxiliary power/ lighting systems. This is were someones inverter, charger or lighting may have
stopped working, the lighbar flattens the cranking battery or a multiplex system has been damaged.
2) Lightbar installation, this is NOT supplying a £20 amber beacon, drilling holes in a vehicle roof and connecting it
to your cranking battery! This is doing the job properly so that you incure minimal intrusion to your vehicle body work, will
not have your cranking battery flattened and can have a simple multiplex system to control more complex requirements.
3) Design and installation of Auxiliary power systems, AKA generating mains 230V AC power safely in your vehicle.
Lightbars and beacons are often a cranking battery flattener
It doesn't matter if you are a Police car, breakdown truck or tree surgeon, your lightbar is there to make you more
visible and should be considered so.
Regular problem number one I get called out to (usually at the start of winter) is a vehicle will
start flattening its cranking battery when its out on a call with its lightbar on, but after jump/bump starting when
it is returned to a workshop the battery shows a healthy capacity.
This is commonly caused by the weather. If you had a battery rated at 12V 100Ahr, it will probably say in the small
print " at 20 degrees C ". This means absolutely nothing to most people, but if I said that some batteries at 0 degrees C
will half there capacity you may see why a problem occures.
Example; Fred goes out in his breakdown assistance van to a call 3.00am in the morning, he turns
the key and the engine limps into life. If his battery was a 150Ahr, due to age, wear and tear, it is down to 120Ahr.
Now if the morning temperature is at freezing point, that will half the 120Ahr to 60Ahr which may be just
enough to start the vehicle. He drives 10 minutes down the road putting some charge back into the battery and gets to
the broken down vehicle in a residential area and puts his vehicle into a fend off position to protect the breakdown. He turns
on his lighbar which (if it has 6 rotators and flashing halogens) draws 30Amps. If his battery is was already at 60Ahr,
take out another 30 Amps for half an hour (another 15% of remaining capacity) while the vehicle is being assessed, that
will take the battery just below its threshold to be able to start the vehicle. Given a bit more time, will also loose power
to the lighbar leaving him cold, immobile and stuck in the dark
Whats the solution? I would recommend a split charger with second battery, this will insure you can always start.
Other people may recommend battery protect system which works out cheaper and will turn off your lightbar when the
cranking battery gets to low. This is fine in principle, but if you are a Police vehicle fending off after an RTA (road
traffic accident) and your lightbar switches off, you instantly loose your visible defence, you then need to run back to the
vehicle and restart the engine as soon as possible.
LEDs are being used more in lightbars which do reduce power consumption substantially, but if you consider an ambulance
with halogen technology blue lights will use upto 60Amps just on its 999 light circuit, you can see why they need atleast
3 batteries onboard.
I do also stock a 'plug in your lighter' magnetic rotator in Blue or Orange, difference with mine is its designed
for speeds up to 130mph!
Most common request for a vehicle installation besides lightbars are for mobile mains power (230V) to be made available.
This may be to run power tools, cooking equipment or a video game, but the 2 key requirements are how much
power for how long. I will keep this as simple as possible using the actual figures only and no full equations.
Example: You have a 1000W (1KW) drill which you need to run at full power for half an hour.
A Quick calculation (not including losses, DC correction etc) to see how much current is being consumed; 1000W
at 230V draws 4.5A (1000/230=4.347) so you can now roughly calculate the correct cable thickness and fusing
from the inverter also the inverter rating itself. You now need to calculate the current drawn through the 12V circuit
from your vehicle to the inverter, so again not including the losses, etc; 1000W being drawn by the inverter at 12V draws
84A (using the above calculation again 1000/12=83.333). We know it will in practice be more than 84Amps as that
is just the power required by the drill, not including the inverter, but we now know we need 84 Amps minimum for atleast
half an hour. Armed with this information you can then make your choice of battery capacity, inverter feed wire size and fusing.
You now know roughly how much power you need and a battery size, but you now have to be able to recharge your new battery.
Split charging is the simple solution for mobile charging. If your vehicle has a 95Amp alternator and it uses 50Amps nominally
for lights, controls, fans etc, you can use the other 45Amps through a split charger to charge up your second battery.
If you know that your drill has used 84Amps in half an hour from your battery and your alternator has 45Amps
available, it will take 2 hours to put 90Ahrs back into the battery covering the energy used by the drill.
Items to consider though are voltage drop down cables (a cable may be rated at 100Amps but you will still
loose voltage over distance), Inverters should be chosen at a minimum of 30% above nominal power consumption, if using power
tools, motors, etc, go for double the nominal power used due to heavy inrush currents.
The above is for use as a rough rule of thumb, if it seems like rocket science, don't attempt it, give us or a
qualified vehicle technician a call! The backbone of a vehicle auxiliary power system is reliability of the installation,
common poorly designed systems just take a feed from the cranking battery to a second battery then connect whatever they need
to the second battery. This will cause problems! If you are using the 1Kw drill in the back of the van, the drill
will not only take power from your second battery, but from your cranking battery as well leaving you unable to start your
vehicle.
Below is how your system should be configured
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Secondary (auxiliary) battery |
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With this system the key part is the split charger, this is not a relay getting a signal from your alternator, but an
intelligent heavy duty contactor (further details can be found from www.intellitecmv.com or www.antares.co.uk). The way this charging system works is that when the engine is running and the battery reaches a healthy
voltage, the split charger will close its contacts to start charging the second battery. If the cranking battery falls below
a safe voltage the split charger breaks contact. This way while the vehicle engine is running the cranking
and secondary batteries will charge, when stationary with the engine off using equipment powered from the second battery,
the cranking battery is disconnected so you may flatten your secondary battery, but still be able to start your engine and
recharge it. Some split chargers will work in reverse as well, so that if you put your secondary battery on
to a mains charge, when it is full, the split charge will then close its contacts and start charging the cranking battery. A
usefull feature on the Intellitec split charger is if your cranking battery is to low to start your engine, it will automatically
close its contact to give you the extra power from the secondary battery. The inverter uses the 12V from the
battery and steps up the voltage to 230V, their is a wide choice of inverters on the market with a lot of false statements
made to sell them. If you need a 500 Watt inverter, make certain it is a 500W continues rating! Some state
500W, but that is the maximum peak power available for 2 minutes while its continues rating is only 200 Watts. Also
watch out for Sinewave against quasi-sine etc. For just power tools, lights etc you can get away with a non-sine wave
unit, but for laptops or sensitive equipment you will probably need a proper sinewave device or you may end up with extra
whistles, blips and bangs.
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| This lot was parked up near a recent job I did, wonder if Top Gear need a sparkie? |
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