auto switch-off a device circuit when full charge

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Metzen

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Hello,
I'd like to ask if you could help me with desiging a circuit that auto switches off a device when 12v in paralel batteries are at full charge, say at 13v, and that starts it over when the voltage drops more than 12v. It should also have some sort of protection against shortcircuits so the device doesn't get busted nor the batteries.
I believe it sounds very simple but the thing is I just don't have the knowledge for figuring it out.
Thanks a lot and farewell.
 
What kind of batteries? Lead Acid?
What battery capacity?
Recharge time?
 
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MikeMl said:
What kind of batteries? Lead Acid?
What battery capacity?
Recharge time?
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Well, I need two circuits of this kind for two diferent projects, but for now I want it for a single 12v 45Ah normal car battery, which I don't know exactly whats it's chemistry, I think it should be the most common one. EDIT: No recharge time, the time needed for bringing it up to 13v again.

alec_t said:
That would bring the voltage close to zero. Do you mean when the battery voltage drops below 12V?
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Yes, excuse my english, I meant below 12v ^^

Thank a lot
 
best practise is to use foldback and 3 stage charger to match chemistry or mfg specs.

Stage 1 : CC ( max constant current allowed) or sometimes C rating in 1 hr.
Stage 2: CV ( Charge voltage once reached current reduces to a threshold of < 10% CC or as desired ) e.g. 13.6~14V for SLA or 14.2 for open cell LA
Stage 3: Float or shutdown after CV current drops below threshold or in case of Open Lead Acid skip this step and leave at CV of 14.2V and current drops to 0 naturally.

This requires just voltage sensing but for other battery types when using LiPo or SLA (Sealed lead acid) or NiMH then current sensing is needed.

Either way, you need short circuit protection. Smart way is sense battery voltage & correct polarity then apply power by relay or MOSFET.

Many ways to implement. You decide by detailed specs.
 
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Is your 'device' a battery charger or something else?
What current does your 'device' draw?
 
Thank you, so many new concepts. Good to know!
I can see this is for finding out the battery specs. Mine is 12v and 45Ah, and I'll assume it's SLA, the other types look expensive and mine it's not.

alec_t said:
Is your 'device' a battery charger or something else?
What current does your 'device' draw?
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The 'device' is a 24V 1.8A DC motor, but, correct me if I'm wrong, the current may be more due to some load.
 
I used a relay that was controlled by a pic, it used the A2D convertor to measure the voltage. The relay turned on the power supplying a "stupid" battery charger when the voltage dropped below a set voltage and turned off the power to the battery charger when full. It was a quick method of charging my trolling motor battery without me constantly visually checking it, I also added a piezo buzzer for when it shut off. I had to add a diode between the charger and the battery due to it back feed the charger when the power was turned off. the pic was powered by an old phone charger placed before the relay.
 
That's sounds just like what I'm looking for. Could you give more details on how to set it up from your own experience or guides I could look into? I'm really newbie, and as far as I have seen this pic is quite complex. Anyway, that gets me on the road! thanks a lot!
 
it works good, it was a temporary fix but heck, I keep using it and yes the pic's take a bit to learn but are very much worth it, and with the pickit2 or pickit3 you can start really cheap. they are good to get to learn and I use them 99% of the time, It's never to late to start to learn them. I started to read these (

also I think you could use a pair of TL431 to do the same thing, I remember reading about a simalar project using them. Doing a search for tl431 on this forum may turn up an idea and circuit
 
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Ideally for outdoor use or wide temp range, Step 2 CV mode must be thermistor regulated to comply with battery specs. because overcharging the voltage generates H2 which with any O2 becomes explosive. So if in a sealed container Teflon plug is used to allow H2 to leak out without letting in H2O vapour.

SLA Float voltages for ref only are here. https://en.wikipedia.org/wiki/Float_voltage
Consult with OEM vendor specs.
 
If I've understood your requirements correctly, this should do the trick :-
 
No that does not do Stage 1 or stage 3
It is just CV hysteresis, which is none of the steps I outlined.

Step 1 requires current sense or a supply incapable of exceeding the charge rate.
Step 3 Float requires Current sense to be below 10% of rated current so that voltage is latched to the float voltage until a load drops the voltage below a specified amount the restart into Step 2~ 3 mode, which may oscillate over a long period, which is ok, not a short period.

This simplest design is a current limited CV set to float voltage.
 
Perhaps the OP should clarify.

I can't imagine why a motor is switched off when the battery is fully charged. I must have missed his requirements.
 
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Tony Stewart said:
I can't imagine why a motor is switched off when the battery is fully charged.
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The intention maybe is that the motor switches on once the charge level exceeds 13V but switches off when the battery volts drop below 12V. In practice there could be problems because of the bounce in battery voltage as the load current changes.
 
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That makes more sense but opposite of what he said.

a circuit that auto switches off a device when 12v in parallel batteries are at full charge,
 
opposite of what he said.
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Yes. The circuit I posted would enable either option if, say, a SPDT relay were used. Perhaps the motor is for a fan which cools a battery charger? Who knows .
 
Thanking your interest and satisfying your curiosity I explain now that the motor spins an alternator that feeds the battery, that's why I need it to stop when it's at full charge. At first I thought of connecting it to a cheap solar charge regulator with it's output connected to the motor and just letting it run forever, but surely the motor would burn down or the system stopped due to the used energy and the charge regulator low-voltage protection, so then I thought to set two separate circuits one for the motor start/stop and another for the alternator to charge the battery and an output for using the energy. Also the the mentioned charge regulator is set to stop output at about 10-11v so the motor would still have enough energy to keep charging. Now the most preocupying question left is if the motor will have enough power to spin the alternator, I haven't been able to do the calculus of cynetic equilibrium... but anyways, I'll give it an empiric try. Good bearings, 12kg of the home-made alternator discs, 1,5 Nm motor's torque, from there on I just can make a guessing. If this works I'm going for a 3kw system. Wish me luck!
alec_t said:
If I've understood your requirements correctly, this should do the trick :-
View attachment 91313
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I can't understand how it works and looks great but for me to build this circuit I'll need explanations on how to connect the microchips, as I've seen they have many inputs-outputs. Also which would be the relay specs? Do you think the motor or the circuit needs a fuse? what max current should it stand?
Joe G said:
and with the pickit2 or pickit3 you can start
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I'll take a look into that!
Thank you!
 
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