Alternative Battery For 12v devices (dash cams etc)

cormie wrote: »

Thanks a lot for the reply!

This is from the ebay page of the battery. It says recommended maximum charge is 3.6a? nothing about volts there? http://ebay.ups-direct.co.uk/public-pic/1wiRRKn_large.jpg

There's some more info here too: http://ebay.ups-direct.co.uk/public-pic/dzSZZ9B_large.jpg

I'm not sure which of the wires was hot but I think it may have only started happening once the aux batteries were flat. Not definite on that though. The wires are pretty small anyway.

Do you reckon I could get away with a charge controller while still allowing the camera to start up as soon as the ignition is on with a flat aux battery, or would the aux battery need to charge for maybe half an hour before the camera will have enough power to operate?

A big Can of worms is when the battery is flat some reason

I did not see any circuit diagram for this set up

the big thing with cars is really AMPS
Amps are simple enough the small lights on cars are about 1/2 amp
car battery of 40 amp will run this lamp for ~80 hours eg 40/0.5= ~80
head lights are about ten amps the two of them so 40 amp battery will mathematically run the head lights for 4 hours ( in reality is less than 1 hour ) as the load is so large it like average human male to carry 100 kilos it wrecks the whole system

Car batteries under very heavy loads heat up and throw away lots of energy as heat much as over burdened human will do

Car flooded acid batteries are best when they have small loads like max ~4 amps load on ~40 amp car battery for short periods like ~2 hours max so 80% the battery power remains behind

The problem is the average medium sized car car has main battery of lead acid flooded type approx ~40 to 60 amps ( can be sealed lead acid SLA flooded acid type) which often reaches full charge at about ~13.5 to ~14 volts

The second battery is what is called a SLA sealed lead acid but critically AGM acid glass mat and it has some important ant differences as it often fully charged at lowwer voltage rates ~13 to ~13.5 volts

With this type of set up in car where voltage spikes come from car generators when both batteries are fully charged these small differences in voltages probably wont show up as issues

When the two batteries are full the alternator wont detect that one battery is very small compared to the other as it will supply very little power to each battery

The fun starts when one battery is much lower voltage and is in this case a lowwer capacity.

The smaller battery recommended maxim charge rate is ~3.5 amps
(in realty these AGM batteries are real robust and can take much more than that for short periods but does reduce life times a tad )

When the normal car with ~40 amp battery starts up if the battery is flat say 10 volts the normal recommended charge rate for the flooded acid type is ~10% of capacity or 4 amp rate .To stuff into a battery at ~4 amps to reach full charge of 14 volts the alternator must produce more voltage than the battery to push the power into the NOMINAL ~12 volt battery.The alternation power source is ~18 volts ( at full speed can be as low as 16 volts on tick over ) and that will is higher than 12 volts so power goes to the lower power source 12 volts
(battery at ~12 volts is empty and ~14 volts is full and ~10 volts is flat. The car battery with engine off under heavy loads like all lights on wipers on aircon on and radio will drop to down ~12 volts if its fully charged and reads ~14 volts at rest )

We can see that large voltage swings are the norm in the system so voltage isn't a exact thing it can float from ~11 to ~16 volts safely .

If the car battery is broken the full 18 volts hits the headlights with 50% more power and the head light bulbs blow very frequently

If the car battery ~40 amps is flat the 18 volt alternator will be much higher power than ~10 volt battery and power from the alternator will rush to the flat battery .
As the battery fill up the heat Resistance from battery will stop the power entering the battery so fast .
When the battery is ~14 volts the heat resistance will completely stop the 18 volts entering the battery .
It similar to bike pump as the Tyre fills with air it get harder to pump air into the very full bike Tyre but the early part of filling the empty bike Tyre goes quickly

Now for practical real world stuff the alternator can supply ~35 amps of power way more than the ~4 amps the battery is supposed to take .
What happens is the flat car battery will soak up the full 35 amps of the power the alternator supplies for some few minutes .
The voltage will rise to ~12 volts .At that time the instead to be ~18 volts and ~35 amp pumping into ~10 volts the extra voltage ~12 volts will block and reduce the ability to pump power into the battery so it will be more like ~12 volts and ~10 amps going in .
Later about half hour when the voltage gets to ~13 volts the Resistance will increase and it will be more like ~13 volts and ~5 amps charging probably
~5 amps is close to recommended ~4 amps

It will then as the voltage creeps up to ~14 volts slow down a lot until finally after two hours total the battery is full at ~14 volts and the charging from alternator will be 18 volts and 0 amps entering the battery . The difference from 18 volts to 14 volts isn't enough pressure or difference to stuff any power into the battery and so charging is effectively stopped

The graph if one existed would show the more the voltage goes up in the recharging the less amps can go into battery until eventually when full no amps go in. It means often that the flat battery t is ~80% full in the first hour of charging and the second hour will only put in the battery the remaining 20 %

The average amps that went in each hour was ~20 amps ~4 times more than the recommended rate .The flooded acid batteries can take this abuse a few times but run your battery down 20 times in row and the flooded acid battery will be lead scrap .

The important thing to see is that for some few minutes when the battery is totally flat the battery was getting ~9 times the recommend charge rate of ~4 amps it was getting some ~35 amps

If the flooded acid flat battery was a ~20 amp battery it would then be getting some ~36 times more than recommended rate as the recommended 10% rate charge rate for that would be ~2 amps

If the flooded acid flat battery was a ~12 amp battery the battery would be getting some ~50 times the recommend charge rate for a flooded acid of 10% only ~1.2 amps


In this case we have a AGM battery that can accept instead of the flooded acid version 1.2 amps it can take ~3,5 amps about ~25% of capacity (for short periods )

If we hit this AGM battery if it ~10 volts and flat with 35 amps from a alternator that is ~10 times it recommended charging rate
Its gonna be problem for certain parts and battery life and its usually the wires that suffer .
As the battery fills up quickly the voltage will rise quickly and soon the amps hitting the AGM battery will drop to more like ~4 amps closer to recommended rates

The AGM batteries ability to soak up incredibly high charge rates for short periods is what protects it from the many baboons that practice the dark art of adding extra batteries to cars

If you add extra battery to the car like this and when the car stops and the design is all batteries stop a simple light weight cable able to take a few amps will be enough to keep the system to run for years without problems as the car alternator will have two batteries similar voltage always full and rarely putting more than a few amps back into batteries.
That would be what most second battery use would work at the second battery to run some light or radio for a hour or two no big deal .
When the car starts again the 12 amp battery might have used 20% of its power some 2.4 amps and the alternator will supply ~5 amps for a few minutes and very quickly that will be less than 1 amp charge rate for a 1/2 hour or hour and its full again

The fun starts of the 12 volt AGM battery is flat or empty and the alternator starts to send some ~20 to ~35 amps at this battery
If its typical set up with ~20 amp wire everthing is gonna heat up .
The battery cant take in 20 to 30 amps probably takes in 10 amps tops .The other extra amps start to have to find a way to get lost so they create heat in the battery plates and if that isn't enough they create heat in the wires.
When things heat up they lose the power as heat .
Now often this heat up time span isn't that long and the battery charges up and the wires and battery cool down and nobody is the wiser .

However if there some extra issue that takes longer to get power into AGM battery the plastic on the wires often start to melt and smoke .
Heated up enough and the story could well will turn into a fire and often that fire will not be covered by fire insurance as it will be classed as act of baboon or similar

Solutions are many

some are real stupid simple take the second battery out of the car plug it into charger and charge it up then the alternator will see two full batteries and not supply much amps and no heating

Create some type of resistor in the circuit to soak up the extra amps while battery is charging .Risks then making alternator run longer time with more amps and making alternator hot and burning out alternator .The alternator can only give ~35 amps for short periods it is only meant to dish out ~12 amps continuous in cooler climates and less in warm climates


Supply a separate independent battery to the Camera .If you have two of these batteries then put one on charge in the house and the other on the camera in the car

Alternatively get a low power 220 volt AC adapt or for the cigarette lighter

Buy with the 12 volt AGM battery a 220 volt plug in the wall charger .Start up the car switch on the AC power plug in the wall charger connect it to 12volt AGM battery and charge it up as you drive
If the AGM battery is very flat the system will still only allow a few amps to recharge it so it wont risk to recharge very fast . This system will work if you do lots of long journeys and battery gets full charge often .This system will fall down if there is lots of short journeys and will require extra charging back at home or adding in second battery at home recharging. In summer adding a small solar panel of 0.1 amp to the AGM battery might sort that problem out

More complex and expensive is to change the wires to the battery to massive wires able to take 35 amps and more because the car battery can also help to charge as the 14 volts from it will want to go into the lower 10 volt smaller battery so possibly more than 35 amps is heading to the small AGM battery .It will require extra circuits to detect the high amps shut everything down .If the circuits fail then your depending on the sheer size of wires to not allow them to become hot enough to melt the plastic .In summer time this might not be enough and fire will become the risk.
Your also often going to need electronic components like diodes which allow electricity to flow only one direction .These electronic diodes often take away voltages and the loss of voltage can mean longer times to charge up but worse can mean the battery never reaches full charge stopping at 80% charge

Alternatively there can be combination systems with temperature or amp flow sensors but that getting very complex

My preference i would be to keep it simple stupid and use the two AGM battery with two charger trick and the AC electric unit in the car .Then its not really part of the car wiring system its accessory like a car cooling fan and like all good accessory to the crime if the car caught fire and was destroyed the blame would be on the OEMS of the components and not on some uninsured baboon electronic head in the street who wired it for you

yes from a electric efficiency point of view increasing 12 volts to 220 and then bringing it back down to 12 volts to charge a battery is brain dead instead to supply 2 amps needed you now need 4 amps to the same work as the losses are large but its pretty baboon proof

If money isn't a problem then you need to make all the electronic circuit solutions that can limit the power to the AGM to max at 7 amps for a few minutes and then to stay below 3.5 amps after that .

The two battery solution has the advantage you can go take the battery out when its 50% empty if it parked for few weeks just rotate them in every couple days .
BALL PARK figure
A battery that is only used partially like when it drops to 80% capacity is recharged will return 1000 cycles
If the batteries in more harsh use never go below 50% capacity continual use they will return some ~500 cycle life or 1000 days use .Two of them will return ~2000 days use .
If there is only one battery always flattened say 80% used every time it will return 200 cycles at say three day to the cycles that 600 days use or with replacing second a 1200 days . (In warm countries with larger solar panels that could work well as the one battery would probably never go below 50%)

The ball park rule for AGM battery WHEN UNDER LOAD eg starp a 20 to 50 watts light load across it and measure the voltage under load .
If the voltage reading is 13 volts then it is full .If the reading is 12 volts its empty .If it 12.5 volts it half full
easy

Good luck