JimHoork said:
When I park my car (no charge power) I leave it for example shut down for two or three days. That is already 62 hours 124 amps if we go out that the car uses his maximum power. But the camara i thing is nominale running at 50% of the max power in parking mode. He will recording when something is moving. So lets say 1A for the hole system in one hour.
so average case usage of 62ah...but potentially 124ah.
If I want park my car easily for three days and I make a battery of 14,8V, I need 100 cells for 75A (single battery 3,7V 3A).
it's more complicated than that.
if you use "3ah" cells, that assumes charging htem all the way up to their max voltage (usually 4.2v) down to their min voltage (usually around 2.8v).
but you wont' be doing that, if you're using 4s and only charging up to whatever the max voltage that comes out of your alternator, when the car is running.
4s, fully charged, is 16.8v, for 4.2v cells. if the alternator goes to 14v, or be generous and say 14.5v, divided by four cells in series, is only 3.625v, so not even 50% charged for many of these cells. assuming you then do use a bms that will allow them to run down to completely empty, you get, at best, half the capacity of the cells. meaning, you only get 1.5ah per cell. so if you need 62ah average, divide 62 by 1.5, and you get 42 (rounded up), meaning 42 cells in parallel. multiply by four, for the four series you need for the voltage, and that's 168 cells, for an average usage 62ah.
now, if you use a boost charger of some type, you could fully charge a 4s pack, and only need half the number of cells vs the above situation.
or, use a buck charger of some type, and use a 3s pack...but then you may need a boost converter on the pack output to be able to use the whole pack capacity for the camera system.
the pack sizes above don't account for pack aging, or higher than average usage, or any other adverse condition that causes the system to need more capacity than you have, that means the cameras might power off before you want them to.
generally, just to account for pack aging, i would recommend adding at least 20-25% more capacity than you calculate for, so that at the end of up to a few years you still have enough capacity to run the system as long as originally designed. if you do that, i think you need about 212 cells.
Than to give power to the camera system I need 12V of power. So than I can add an ''buck converter'' (jonyjoe303) en lower the voltage to 12V for the load.
you should check the specs for the camera system, but if it si made for automotive use, it's designed to handle a range of probably 10v to 13v at least, and probably as high as 15v. but you should check the specs first.
if you do have to use a dc-dc converter, keep in mind that they aren't 100% efficient. so you need a bigger battery by whatever percentage the converter wastes power, to get the same runtime with the converter that you'd get without it.
i'm a bit on the tired side right now, so you should check my math below.
Than finally I was thinking what is better: power up the battery on the idea of jonyjoe303 and set-up a ''buck converter'' to charge or the same installation but with the charge cables on the car's protected accessory point. But then the question is I thing is the protected accessory point gives enough power to charge a 75A battery?
a slower charge can be better for cell longevity, but if you use a charger system that only allows half the cell capacity in a, or less, per parallel cell, it would take a little more than two hours (or more) to charge the pack. if you need the pack to charge faster, it dpeends on the cells you choose for the max rate. with typical cells, you might be able to do as fast as half an hour charge (at twice the cell capacity in a, times the number of parallel cells).
that's assuming you're charging to the full voltage of the cell. if instead you only charge to the voltage the alternator gets, the charging will take less time.
if the pack is say, a 124ah pack (assuming you're using only 62ah of it), then you could charge it in say, 15-20 minutes, but taht would be at almost 250a.
what kind of charger you should use...that i don't know. i think that you probably could use just the accessory point...except that if you need that much charge current, you can't--you *would* have to connect directly to the car battery. high currents make things complex.
it also means you couldn't charge thru the bms itself, or else you'll need a heck of a bms to handle that kind of charging current--much much bigger than you actually need for the load itself.
someone else hopefully will re-run my numbers to see if this is correct, and help you figure out a solution....
