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24v truck to 12v charging solution needed

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cf_corp

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Hi folks,
Ive got a truck with a 24vdc charging source and I want to install a small security system that runs on 12v with a 12v 1.5Ah backup battery, the system which ive made myself works perfect on the 12v battery i have. Whats the best/cheapest solution to ensure that the 24v could be converted down to charge a 12v battery and regulate the charge so theres no nasty overcharging, over discharging etc? anything you guys know of on ebay? I was thinking a dc-dc converter with 12v charge regulator, I was looking at those solar chargers but they seem to be set up in such a way that if the charging source is 24v, then the battery should be 24v with the output load at 24v. Could it work if I got the solar charger, run it at 24v and put 24-12v dc-dc converters on the battery output and load output. I am also just wondering about the 12v dc-dc output whether it can charge the battery as i am thinking the output will only be 12v. any simple effective solutions? Thanks folks
 
How much current does your security system draw? If it's very little then a simple trickle charger may suffice.
 
hi alec_t,
shouldn't be any more the 2-3A current draw with alarm activated, thats for the sounder and the strobe the battery is only for backup reasons etc, you know if the 24v supply was to be cut off, constant power in idle mode would be less than 100mA.
 
Are you willing to kill your main 24V vehicle (starting) battery by having a 100mA constant draw on it 24/7? A 100mA constant draw will discharge a 100Ah battery to the point it will begin sulphating in just 4 days. It will kill it stone dead in less than a month.

What kind of alarm requires 100mA in standby mode? Most commercial alarms are <10mA.
 
Years ago I did auto design for 24V trucks. The trucks had two 12V batteries.
The two way radios work on 12V. It is connected to the bottom battery at 12V.

I could connect your alarm to the bottom 12V battery.
 
sorry, i meant the power draw would be around 10mA not 100mA, The alarm system will be working from the trailer, which the one i have has a constant 24v supply,
 
could i use an adjustable dc-dc step down, adjust it to around 16v, use the 16v to supply a charge regulator to charge the 12v/1.2Ah battery? the dc-dc can supply 2A rated, or 3A with a heatsink. 32-48watts at 16v. the cahrge controller I was looking at can charge at up to 1.5A.
 
The ultimate simple charger: a 750Ω resistor connecting the two batteries. That gives you 10mA for the alarm standby current and ~5mA to keep the 12V battery topped up (you'd need to give it a full charge elsewhere before use, or if the alarm actually tripped). A transistor and ~13V zener could be added to cut off the current if you're really concerned the 5mA could cause over-charging.
 
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Years ago I did auto design for 24V trucks. The trucks had two 12V batteries.
The two way radios work on 12V. It is connected to the bottom battery at 12V.

Doesn't that imbalance the state of charge of the two batteries? Since the charging current from the alternator flows through the batteries in series, doesn't that chronically overcharge the upper battery?
 
Doesn't that imbalance the state of charge of the two batteries? Since the charging current from the alternator flows through the batteries in series, doesn't that chronically overcharge the upper battery?
If the unbalance is only a couple of amps there is no problem. If the batteries are anywhere near the same charge there is not a problem. If the top battery is at 95% and the bottom battery is at 85%, It takes more current by far to go from 95% to 100% than to go from 85% to 90%. So as you approach 100% the batteries will try to re-balance.

On 24V systems that I know of the headlights are 12V, all the lights are 12V. The radio is 12V. I have used systems with a 'balancing' circuit that forces the charge to be the same. I have also used systems with out the balancing circuit. It will work as long as you don't allow the batteries to discharge below 50%.
 
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