I need to build a float charger as the car is used only few time.
It stays in a garage at the average temperature of 27°C /year
I have a standard car lead acid battery 12V 62AH.
The battery will be not disconnected during float charging.
I'm thinking of a circuit with constant voltage of about 13.65 Volt and a costant current of 1.4ma/AH so 1.4*62= 87ma
Thus the charge should be 100% safe as rated 0.087A/62AH < 1/500 Battery Capacity << 1/10.
Are the numbers correct? I could even rise the current to 100ma for self-discharging plus stand-by consumes.
So a costant voltage circuit of 13.65Volt , limiting max current at 87mA (or 100ma). I could use a L200 or two 78xx voltage regulators in series one as costant current and the other as constant voltage.
It stays in a garage at the average temperature of 27°C /year
I have a standard car lead acid battery 12V 62AH.
The battery will be not disconnected during float charging.
I'm thinking of a circuit with constant voltage of about 13.65 Volt and a costant current of 1.4ma/AH so 1.4*62= 87ma
Thus the charge should be 100% safe as rated 0.087A/62AH < 1/500 Battery Capacity << 1/10.
Are the numbers correct? I could even rise the current to 100ma for self-discharging plus stand-by consumes.
So a costant voltage circuit of 13.65Volt , limiting max current at 87mA (or 100ma). I could use a L200 or two 78xx voltage regulators in series one as costant current and the other as constant voltage.
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The internal leakage of the battery may well draw more than your 87ma when you apply 13.65 volts, particularly if the battery is warm.
That said, feeding that kind of current into the battery will prevent it discharging.
I can't remember the numbers now (and it depends on the specific gravity of the acid), but you also might find that 13.65v is actually a little to high for 24/7 application in a warm climate.
That said, feeding that kind of current into the battery will prevent it discharging.
I can't remember the numbers now (and it depends on the specific gravity of the acid), but you also might find that 13.65v is actually a little to high for 24/7 application in a warm climate.
Thanks
With limiting max current to 87ma, battery cannot suck more than 87ma even if partially discharged or warm. It will just require more time to get full charge.
At 80% of capacity it should require:
62Ah*20%=12,4Ah
12,4Ah/0,087A=143h=6 days
...if, I'm correct...
i will check better the safest float voltage...
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Most automatic battery chargers charge up to 14Volts and then float at 13.6Volts. Never charge at more than 10% of the A/H rating of the battery.
Holding the battery at 13.6Volts will not damage in any way once charged. There is no need for silly milliamp limiting.
I recomend using the cigar lighter/DC socket to charge with.
Holding the battery at 13.6Volts will not damage in any way once charged. There is no need for silly milliamp limiting.
I recomend using the cigar lighter/DC socket to charge with.
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In practice 87ma would probably maintain a fully charged battery state on a good battery, and again in practice, it would not be able to bring a discharged battery back to full charge.
After my experience, a correct voltage is 13,8 V, and max current 150~200 mA. At 68 mA I think the battery voltage will go down, because of both self-discharge and equipement (calculators, radio and so on). But its better to try !
Floating of battery which is connected to an equipement is always complicated matter.
Floating of battery which is connected to an equipement is always complicated matter.
To maximize the battery life the float voltage should be temperature compensated to within 50mV. Google will find you some technical references.
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery
Ok thanks, I have done this ULTRA SAFE FLOAT BATTERY CHARGER and now is working cold after few days (battery is quite new).
It has 4 fuses, one fast on the mains, one delayed on the output, two inside the transformer.
Everything is dimensioned at least 5 times the necessary, as it is a circuit connected forever, with the battery plugged onto the car.
I have read that to prevent sulfation is recommended in any case to unplug the float charger at least once every 60 days during seasonal storage. Allow the battery to rest for a couple of days, and then plug the charger in again.
I have measured my car stand-by current of 12mA, and I have calculated a self-discharge of nearly 5mA (80uA/AH)
I have measured (but with battery unplugged):
12.95V 116mA
13.15V 109mA
13.20V 91.5mA
13.22V 86mA
13.28V 79mA
13.65V 31.4mA
13.84V 17.5mA
quite close to LTspice simulation
Attached photo, comments, circuit and LTspice simulation.
I have tested several rectifiers before choosing these ones, to find the optimal sum of voltage drops across the forward-biasing at such low currents.
Comments are very welcome, even if I have already done!
Attachments
That looks quite good. Only concern would be that 13.8 volts is still a bit high for 27C ambient.
Yes, but is at 17mA, thus 1/3600 of C !!!!
It should just cover self discharge plus stand-by consumes...
The current you measure into the battery is a variable. What matters is you maintaining 13.8 volts across the terminals which imo is to high for float charging at such a high ambient temperature.
If you measure the battery terminal voltage after standing disconnected for a few hours then you will probably find it at around 12.5v (at a guess). And that battery is fully charged. So if you apply anything over that voltage to the battery you are maintaining its charge. If the self discharge rate increases (and its hugely variable over time/temperature and aging of the battery) then the terminal voltage remains the same but the charger supplies more or less current.
I'd stick to around 13.5 volts and no higher. You can easily trim the voltage by using a 7812 rather than a 7812 and adding a variable resistor in series with the ground pin.
If you measure the battery terminal voltage after standing disconnected for a few hours then you will probably find it at around 12.5v (at a guess). And that battery is fully charged. So if you apply anything over that voltage to the battery you are maintaining its charge. If the self discharge rate increases (and its hugely variable over time/temperature and aging of the battery) then the terminal voltage remains the same but the charger supplies more or less current.
I'd stick to around 13.5 volts and no higher. You can easily trim the voltage by using a 7812 rather than a 7812 and adding a variable resistor in series with the ground pin.
Hi your 12V 'battery maintainer' project looks very good! thank you for sharing
IMO anything ~13.6-13.8 is fine for temperate climates. The fact that the battery current is much less than 100 mA indicates fully charged AND in great heath. Adding an indicator light or meter showing that it's not in constant current would be ideal.
In my experience w/ flooded lead cells the most important thing for long life and to delay sulfating the lead plates is keeping the electrolyte from stratification or forming layers. You can help this by applying an 'equalization charge' every other month / this is stirring it with bubble action. I let run for about a hour for anything ~50-80 Ahr. you can read about ithere
I now only use a battery maintainer if I absolutely can not be there to check it over a long period. I babysit two small batteries sized like yours. the length of charging is dependent on battery size mostly.
IMO anything ~13.6-13.8 is fine for temperate climates. The fact that the battery current is much less than 100 mA indicates fully charged AND in great heath. Adding an indicator light or meter showing that it's not in constant current would be ideal.
In my experience w/ flooded lead cells the most important thing for long life and to delay sulfating the lead plates is keeping the electrolyte from stratification or forming layers. You can help this by applying an 'equalization charge' every other month / this is stirring it with bubble action. I let run for about a hour for anything ~50-80 Ahr. you can read about ithere
I now only use a battery maintainer if I absolutely can not be there to check it over a long period. I babysit two small batteries sized like yours. the length of charging is dependent on battery size mostly.
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