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Battery Charge Sensing

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jada88

Member
Hi Everyone
I am new to this forum and relatively new to electronics. I have looked at various forums and web sites for a circuit to switch on a diesel generator to charge a 24volt battery bank. The switching on and off of the generator I have no problem with, it is the sensing of the depleted and full charge of the battery I am having trouble with. I was thinking of using a LM3914 I/C to monitor the battery charge level with a signal from the low side, say 21volts and a signal from the high side, say 29volts. These signals will drive 2n3906 transistors which will in turn drive a relay coil. Other members of this forum may or will have a better ideas and a circuit that I can use. I look forward to your comments and advice.

Regards
jada88
 

ericgibbs

Well-Known Member
Most Helpful Member
hi,
The LM3914 is often used in this way.:)

It can also be done using comparators, say a LM393 etc.

What help are you asking for.?
 

jada88

Member
Hi Eric
I am looking for a circuit to turn on a relay when the battery is depleted to say 21Volts,
and turn it off when the battery is fully charged. I realize that the voltages can be adjusted to manufacturers recommendations

jada88
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi Eric
I am looking for a circuit to turn on a relay when the battery is depleted to say 21Volts,
and turn it off when the battery is fully charged. I realize that the voltages can be adjusted to manufacturers recommendations

jada88
hi,
I would expect that you would require that when the voltage sensing ciruit detects <=21V, it energises a relay and the relay to remain energised until the sense voltage is =>31V.

Will this voltage sense relay just give a warning or will it switch the generator ON/OFF.

If its just a warning and say, it gives a low voltage warning at <=21V, will you manually cancel the alarm and reset it to give a warning when the sense voltage =>31V [ and vice versa]

Do you see the point I'm trying to clarify.?:)
 

tcmtech

Banned
Most Helpful Member
I like using this circuit for toggling actions based on voltage differences. I have a basic dicription of how it works on the grid tie inverter thread.
 

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chemelec

Well-Known Member
Hi Everyone
I am new to this forum and relatively new to electronics. I have looked at various forums and web sites for a circuit to switch on a diesel generator to charge a 24volt battery bank. The switching on and off of the generator I have no problem with, it is the sensing of the depleted and full charge of the battery I am having trouble with. I was thinking of using a LM3914 I/C to monitor the battery charge level with a signal from the low side, say 21volts and a signal from the high side, say 29volts. These signals will drive 2n3906 transistors which will in turn drive a relay coil. Other members of this forum may or will have a better ideas and a circuit that I can use. I look forward to your comments and advice.

Regards
jada88
You Can't really tell if a Lead acid battery is Fully Charged, By just Measuring the voltage.

In Theory for a 24 volt battery, it is Fully charged when its at 29.2 volts.
But it also depends on What voltage your generator is putting out and what Current is Being Drawn into the battery when at that voltage.

Example: If your generator Puts out a Somewhat Higher Voltage than 29.2 volts.
Lets say 35 Volts, Than the battery will reach 29.2 volts, long Before it is Fully Charged.

Example 2: If your generator puts out exactially 29.2 volts, the voltage measurement will work Good, Assuming the Battery is in Good Condition, but it may take TWO DAYS to get charged to that level.

You actually need to measure that 29.2 Volts, But you must also Know the Current Draw is LESS than about 1 Amp.
 

Sceadwian

Banned
I thought no load voltage was a relativy accurate way of measuring a lead acid's capacity?

Deep Cycle Battery FAQ

State of charge, or conversely, the depth of discharge (DOD) can be determined by measuring the voltage and/or the specific gravity of the acid with a hydrometer. This will NOT tell you how good (capacity in AH) the battery condition is - only a sustained load test can do that. Voltage on a fully charged battery will read 2.12 to 2.15 volts per cell, or 12.7 volts for a 12 volt battery. At 50% the reading will be 2.03 VPC (Volts Per Cell), and at 0% will be 1.75 VPC or less. Specific gravity will be about 1.265 for a fully charged cell, and 1.13 or less for a totally discharged cell. This can vary with battery types and brands somewhat - when you buy new batteries you should charge them up and let them sit for a while, then take a reference measurement. Many batteries are sealed, and hydrometer reading cannot be taken, so you must rely on voltage. Hydrometer readings may not tell the whole story, as it takes a while for the acid to get mixed up in wet cells. If measured right after charging, you might see 1.27 at the top of the cell, even though it is much less at the bottom. This does not apply to gelled or AGM batteries.
 

Boncuk

New Member
I think it's best to determine the battery charge status by disconnecting it from the charging source and simultaneously discharge the battery (partly) by charging an electrolytic cap while monitoring the battery voltage.

If the battery is partly charged the voltage will be reduced significantly when charging the cap. If the battery is fully charged almost no voltage drop will occur.

The capacitance selected should be within battery parameters. E.g. it is nonsense to discharge the battery into a 100,000µF cap if its nominal rating is just 2.5Ah, but 4,700µF would be a more realistic value.

To make it simple concerning wiring and logic I suggest to use an MCU.

That way charging and discharging cyles would depend on the battery status, meaning an almost depleted battery would have long charging cycles interrupted by occasional discharging cycles. If the battery is fully charged the discharge pulse will not lead to a new charging cycle since the minimum voltage is still being maintained.

Boncuk
 

MikeMl

Well-Known Member
Most Helpful Member
This thread outlines a simple voltage detector which has adjustable Cut-in and Cut-out. The circuit I developed for that application was for different voltages, so I have modified it for your application: As depicted, the trip points are 24.0V and 29.2V. The trimpots will fine tune these.

I agree with Chemlec. You cannot terminate charging of a lead-acid battery by simply measuring the voltage during charge. After the battery reaches ~29.2V, you should be monitoring the current flowing into the battery, and then terminate the charge when the battery current drops to ~25% of what it was when the battery first reached 29.2V. This is called the three-cycle charging algorithm.
 

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MikeMl

Well-Known Member
Most Helpful Member
I thought no load voltage was a relativy accurate way of measuring a lead acid's capacity?

Deep Cycle Battery FAQ
To determine a battery's state of charge from its open-circuit voltage, you have to disconnect the battery from charger and loads, let it sit for 24 hours, then measure its terminal voltage with an accurate high-impedance voltmeter.
Hardly the sort of thing you can do while charging:D
 

jada88

Member
Thankyou for all your replies to my post Battery Charge Sensing, it looks like I have a lot to think about. Answering Eric's question, I just want to switch the generator ON or OFF.

jada88
 

MikeMl

Well-Known Member
Most Helpful Member
I think it's best to determine the battery charge status by disconnecting it from the charging source and simultaneously discharge the battery (partly) by charging an electrolytic cap while monitoring the battery voltage.

If the battery is partly charged the voltage will be reduced significantly when charging the cap. If the battery is fully charged almost no voltage drop will occur...
Boncuk, do you have a reference to any published articles about this technique?
 
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ericgibbs

Well-Known Member
Most Helpful Member
This thread outlines a simple voltage detector which has adjustable Cut-in and Cut-out. The circuit I developed for that application was for different voltages, so I have modified it for your application: As depicted, the trip points are 24.0V and 29.2V. The trimpots will fine tune these.

I agree with Chemlec. You cannot terminate charging of a lead-acid battery by simply measuring the voltage during charge. After the battery reaches ~29.2V, you should be monitoring the current flowing into the battery, and then terminate the charge when the battery current drops to ~25% of what it was when the battery first reached 29.2V.
.
Hi Mike,
Is the 7805____.lib for LTSpice open source.? if yes any chance of a copy/link.
 

MikeMl

Well-Known Member
Most Helpful Member
I got it off the LTSpice Yahoo Group files section. It is a netlist-based library and covers 7805-7812-7815. They have the symbols, too.
 

ericgibbs

Well-Known Member
Most Helpful Member
I got it off the LTSpice Yahoo Group files section. It is a netlist-based library and covers 7805-7812-7815. They have the symbols, too.
Many thanks, I'll sign up.:)
 

k7elp60

Active Member
Thankyou for all your replies to my post Battery Charge Sensing, it looks like I have a lot to think about. Answering Eric's question, I just want to switch the generator ON or OFF.

jada88
One way to tell if the battery is fully charged is to monitor the charging current and when it drops to .01C in the cyclic method(charge voltage between 2.4 and 2.45V per cell). At this point the charger should be removed, or the battery can be overcharged.

Others have already stated the state of charge can be determined by a voltage check without a load and 24 hours after charging.

A dual comparator can be used to detect low voltage and also when the current drops to the .01C level.
 

Boncuk

New Member
Boncuk, do you have a reference to any published articles about this technique?
Sorry, there are no articles about it.

I just built a circuit for NiCads some 30 years ago. Being concerned with long flying times of RC-model aircraft I couldn't afford fitting a not fully charged battery in the plane.

I just made the circuit hard wired with a minimum time interval to switch from charging to discharging and back using relays.

In the beginning of a charging phase the relays cycled at one minute intervals. With progressive charging the duration between cycles increased to come to a full stop when the battery was fully charged.

If you want to design such a circuit don't forget to slowly discharge the electrolytic cap while the battery is being charged. (to make the cap ready for another test measurement)

Boncuk
 

smanches

New Member
PWM charge controllers measure the voltage during the off-time of the switch. If the battery was under heavy charge, the drop from the charge voltage to the "disconnected" voltage will be much greater than if it was under light charge. It directly relates to the amount of current flowing, without needing a current sensor.
 
hi eric i am new member ,woud you like to help me to find or get circuit diagram for diesel battery charger 12v,150 A because i need it seriously ?,,,,,,,,,,
if any body have this circuit ,help me?
thanks
 

fernando_g

New Member
I like using this circuit for toggling actions based on voltage differences. I have a basic dicription of how it works on the grid tie inverter thread.
LONG LIVE DISCRETE COMPONENTS!
I like your idea of using the SCR as a latch.
 
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