| Why
do my batteries run down so much faster than I expected. I’ve
done all the sums very carefully (including following advice in
magazine articles - and it just doesn’t work out). Many others
I talk to say the same. Also why do batteries fail so quickly? |
| Non-technical
journalists writing on this subject usually make the reasonable
(but quite wrong) assumption that a battery charged by a vehicle
system can deliver what it says on its side, i.e. that a 100 amp/hr
battery is good for about 100 amp/hrs. But it’s not - and
for two main reasons.
Firstly a standard vehicle charging
system will not (and cannot) charge a conventional lead acid battery
(either starter or deep-cycle) beyond 70%. With big battery banks,
65% is more common. For this reason alone, that 100 amp/hr battery
is good for only 70 amp/hr - and that’s when it’s new!
Secondly a battery is effectively
useless (because things stop working) when it’s down to 20-25%
charge (about 11.8 volts). So at most we now have 70 amp/hr minus
20 amp/hr. Which is 50 amp/hrs.
The reason batteries fail so quickly
is that they cannot be regularly discharged below 50%
without drastically shortening their
life. Discharging until they are ‘flat’ is not maltreating
them - it’s murdering them! This is not my theory - battery
makers worldwide agree on this 50% level and have done so for the
better part of 100 years.
With a 100 amp/hr battery this leaves
a mere 20 amp/hrs available. Discharging to 40% (which limits battery
life to about 250 charge/discharge cycles) still provides a mere
30 amp/hrs. Routinely discharging them until they no longer drive
the fridge (about 11.8 volts), reduces their life to less than 100
charge/discharges cycles.
One hesitates to say this, but a great
many published articles on this topic are seriously misleading. |
|
Are there any ways of charging a battery beyond 70% from a vehicle
charging system? |
| There are two.
One is to use a three-step voltage regulator (and preferably a larger
alternator). The second is to switch to AGM or gel cell batteries
(but you MUST change both starter and house batteries). Both these
types of battery charge close to 100% from a standard charging system.
|
| Why
use deep-cycle batteries: mine go flat every night or two anyway.
I get much the same life from much cheaper starter batteries. |
| You are almost
certainly undercharging and grossly over-discharging your batteries.
No conventional lead-acid battery (no matter its quality) will withstand
this abuse. Deep cycle batteries used correctly last up to seven
years. But if used as you are doing you might as well buy the cheapest
batteries you can find. |
| For a large
amp/hr capacity 12 volt system should I use two 75 amp/hr 12-volt
batteries in parallel, or two 150 amp/hr six-volt batteries in series.
Which will give the most power? |
| It is theoretically
better to have two six-volt batteries in series, but in practice
it’s OK to parallel two batteries of identical voltage, amp/hr
capacity, type and age. Watts (the unit of energy) are amps X volts,
so the energy storable is identical with either configuration. |
| I check my
house batteries with a voltmeter morning and night - but I keep
getting misleading readings. What’s happening? |
| The energy held
by a battery is a function of electro-chemical reactions between
the electrolyte (battery liquid) and the plates. House batteries
have a small number of thick plates and hence the electro-chemical
interchange is very slow. Unless the battery has ‘rested’
for a day or two, the voltage you are measuring primarily reflects
the condition of the surface of the plates - it tells you next to
nothing about the true state of charge ‘deep within’.
A virtually ‘flat’ battery may show 12.8 volts for some
time after being charged at a high current for only a minute or
two. Conversely, a charged battery may show as close to discharged
for some time after a (say) a microwave oven has been used. |
How can remaining battery charge be accurately
measured?
|
| By taking a voltage measurement after
the batteries have rested for a few days (which is hardly useful!).
Or by monitoring current in and current out, deducting system losses,
and displaying the result as a percentage of charge. These functions
are included in many three-step regulators and also solar regulators. |
| How can caravan battery charging
be improved? |
| Inadequate battery charging is
common with caravans because the batteries are often 10 cable metres
or more away from the charging source. Battery charging is extremely
voltage sensitive and even a 5% voltage drop reduces charging by
up to 30% or more. Most people (even auto-electricians) hugely under-estimate
the cable size required to minimise voltage drop. A simple formula
to calculate this is: Drop in volts = (L x I x 0.017) divided by
A.
L = cable length in metres, I = current (in amps),
A - cross sectional area of cable in mm.sq.
The maximum acceptable voltage drop is 5% - but 3%
is the target to go for.
Most caravans and their towing vehicles have totally
inadequate cable sizing. There may also be large losses unless earth
return connections are clean and secure. This can be avoided by
using a second cable instead of an earth return - but this requires
extremely large diameter cable (as the cable length is doubled).
Whilst there is a case for using dual cable (instead of earth return)
to minimise electrolytic reactions between dissimilar metals, it’s
a matter of weighing up the possible consequences of that (and the
need for really sound earth connections) against the need for even
heavier cable.
Another solution is to use Arrid’s TwinCharge
Battery Charge Controller. This device increases the charging voltage
to the caravan batteries only. There are also ways of doing this
using suitably set-up three-step regulators etc.
Another way, for people familiar with vehicle electrics
by using a series diode to fool the voltage regulator into increasing
the charge rate (by 0.6 volt) to feed the caravan battery, but connecting
the vehicle battery via a paralleled diode splitter to drop the
starter battery charging voltage back to where it was before.
The drawback with this method is that it’s all
or nothing - it works best where there’s 0.6 volt drop on
the line to the caravan battery, and could overcharge that battery
if there’s less than 0.6 volt drop. It’s other drawback
is that it only works with (mainly older) voltage regulators that
sense voltage directly from the battery, most modern regulators
are inbuilt into the alternator and sense the reference voltage
internally. |
| My electric refrigerator does
not work very well and I’m getting conflicting advice on correct
cable sizes. An auto-electrician says 2.5 mm/sq is required, but my
fridge handbook says 2.00 mm/sq is fine. |
| You probably have an Electrolux
fridge. Electrolux used to advise that 2.00 mm.sq was fine, but
have recently revised this to 2.5 mm/sq. It all depends on distance
though - you may need 4.00 mm.sq or even thicker if the fridge is
any distance away from the battery.
Use the formula given a few questions above and work
out the cable size needed for not more than 3% voltage drop (e.g.
0.36 volts in a 12 volt system). |
| Do solar panels really produce
what their makers claim? |
| The industry has a global rating system
that does not reflect typical usage, nor reflects usage in 12/24/48
volt systems. In practice, an 80-watt panel will produce about 58
watts in a caravan or motorhome installation. Most have a little panel
on the back advising something like that. The reasons are complex
(and explained in my book). |
| Many people say Uni-Solar panels
produce more than most others. Is this true? |
| These panels use a different technology
from most. They are less affected by shadowing, and hardly at all
by heat (they actually produce a tad more as they heat up). In very
hot places, a 64 watt Uni-Solar panel produces about the same as an
80-watt most anything else. This advantage however is all but lost
in colder places. Their downside is that Uni-Solar panels are about
30% larger per watt. They are great for hot places where there’s
plenty of room. I have 28 of them on my roof of my all-solar home
north of Broome. |
| I’ve just checked the output
of my new Uni-Solar panels. They produce about 10% more than the maker
claims! Why the modesty? |
| A quirk of the technology used is
that cell output is up to 10% higher during the first few months.
Output eventually falls - the panels are rated at this lower figure. |
| How many panels do I need to drive
my fridge, lights etc.? |
| It is not possible to give a generalised
answer, except that it’s always better to have more panels and
less battery capacity than vice versa. Full details of how to work
it all out are in my book ‘Motorhome Electrics - and Caravans
Too’. There’s a useful but less detailed guide in ‘The
Campervan and Motorhome Book’. |
| What are the differences between
fuses and circuit breakers - which should be used where? |
| Fuses are intended to protect electrical
appliance from further damage or catching on fire in the event of
overloading or internal faults. They should be located as close to
the appliance as possible (or within it). Circuit breakers are intended
to protect cables feeding appliances, or groups of appliances, from
overloading in the event of short circuits downstream from that circuit
breaker. In some instances, fuses are used in place of circuit breakers,
but usually to save cost. It is not good electrical practice to do
so. |
| I have a caravan that’s been
imported from New Zealand. An electrician says that it must be rewired
to Australian regulations. Is this really necessary? |
| Absolutely! There are differences
such as the local requirement for double pole switching and RCDs (Residual
Current Devices). Further, the NZ practice of connecting the neutral
line to earth within the vehicle must not be done in Australia (as
it affects the operation of protection devices in the supply line).
Hence, this link must be removed. The actual wiring within the vehicle
is probably fine, but all power points etc. will need to be changed.
Consult an electrician experienced with caravan/motorhome wiring.
|
| Someone told me that microwave
ovens draw a lot more power than the label on them suggests. Is this
really true? |
| Yes it’s only too true! Because
of prior experience with things like light globes, people reasonably
assume that a (say) 100 watt something draws 100 watts of electricity.
Whilst that’s true of many electrical appliances it doesn’t
apply to electric motors, microwave ovens and a few other such things.
A watt is also a unit of work done. A typical microwave oven’s
rating of 800 watts is a measure of the effective heat that it produces.
In doing so it typically draws about twice that amount (about 150
amps at 12 volts) Because of a quirk of battery behaviour, the depletion
effect on the battery is even greater - and the total effective draw
is about 2000 watts. Ten minute’s oven usage thus consumes about
half the total energy used by many caravans or small motorhomes in
a day! |
| I’ve heard that putting an
Aspro tablet in a starter battery revives it sufficiently to start
the engine. Is this true - if so why? |
| It can sometimes be true. It can
also work with Panadol - and even performing Reiki! But it doesn’t
work for the reasons believed!
It works because there’s a delay in the electro-chemical
reactions within batteries (with starter batteries it’s only
a few minutes). The battery revives despite the Aspro or Panadol
- not because of it. It works because it usually takes a minute
or three to go off and find those substances - and the battery revives
during that time. The Reiki technique involves laying hands on the
battery for 10-15 minutes - so it revives even more. The effect
would be just the same if you’d sworn at the battery in Swahili
for the same length of time. It’s the time delay that does
the trick! |
| There’s a lot of conflicting
advice on caravan and motorhome electrical systems. Why is this? And
why should I trust what you say and write if it conflicts with other
opinions? |
| This is a fair and
reasonable question. Much advice is given by well meaning journalists
who have no expertise in this field. Vehicle electrics is well short
of rocket science, but some things that seem obvious (like battery
capacities and microwave oven ratings!) are not. They are real traps!
I was originally an engineer working in automobile
research (where one has to know electrical and electronic as well
as mechanical engineering). I subsequently became a technical writer.
My books and articles are not ‘opinions’. They are just
plain-English versions of what you’ll find in text books in
the various fields I write about.
Most of the stuff has been known for yonks and is
really basic! For example, much of what I have written about lead
acid batteries has been known since the 1950s and will be found
in any comprehensive battery text book from there on. For later
stuff like AGM batteries, I research the field, and seek advice
from manufacturers’ engineering staff. But never their marketing
people!
The best way to evaluate advice (and advertising claims)
in this field is to grasp the fundamental principles yourself. It
rapidly becomes clear what is nonsense and what is not! |
