# 12volts protection circuit and low battery indicator with only the same battery

#### christiannielsen

Joined Jun 30, 2019
337
Hello,

I can find a lot 12 volts battery protection circuits to protect against both over charging and over discharging.

But what if the charging power source is another battery and therefor the power/reference source is not fixed?

I find it hard to make protection circuits when another battery is the power source as it is also hard to make low battery level indicators with only the battery in question. Because the source battery is discharging so I have no fixed reference voltage.

Basically I have a car battery to charge a second though yet smaller car battery. I want to protect my car's battery from over discharging while charging the second battery and I want to protect the second battery from over charging. I would also like to have a low batt indicator for the second battery with only the second battery as power source for that circuit.

Does it make sense?

Maybe I dont know what to search for to find circuits like that. Could anyone point in the right direction?

#### LowQCab

Joined Nov 6, 2012
2,171
How are the 2 Batteries connected together ?
Why are You using 2 Batteries ?
What is the purpose of the smaller Battery "
Why is the smaller Battery not being automatically charged by the Car's Alternator ?
What is the size of the smaller Battery in Amp/Hours, or mAh ?
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.
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#### Alec_t

Joined Sep 17, 2013
12,894
Basically I have a car battery to charge a second though yet smaller car battery. I want to protect my car's battery from over discharging while charging the second battery and I want to protect the second battery from over charging.
If both batteries are the same nominal voltage (12V) and chemistry (lead acid) the larger battery can't overcharge the smaller battery (but might need to limit charge current).
Sounds like you need two circuits: one to prevent over-discharge of the larger battery, the other to provide the state-of-charge indication for the smaller battery.

#### christiannielsen

Joined Jun 30, 2019
337
Two circuits sounds right. How do one make a circuit to protect the car battery from over discharging when the reference voltage/power source is decreasing?

#### Irving

Joined Jan 30, 2016
2,775
Voltage indicator for 2nd battery: LED is on if volts>11.0v (pot approx in centre)

#### christiannielsen

Joined Jun 30, 2019
337
Voltage indicator for 2nd battery: LED is on if volts>11.0v (pot approx in centre)
Thank you! I am afraid I have to order the TL431 now. I'll get back to you when I've tested it. Will this work properly if the second battery is fully charged (let's say 13.5 volts) and discharged (let's say 9 volts) or do the reference voltage have a minimum voltage?

#### Irving

Joined Jan 30, 2016
2,775
a circuit to protect the car battery from over discharging
What do you mean by 'over discharging'? whats the basis of the test?
What sizes are the batteries?

If the test is voltage (very crude) then the same circuit as above could be used with the addition of a P-channel MOSFET. Here the MOSFET is on, delivering charging current when V(batt)>12.5v

#### Irving

Joined Jan 30, 2016
2,775
The issue with what you are proposing is that the second battery will never be properly charged so its operational time will be short and its lifespan compromised - SLA are happiest when fully charged. A SLA battery needs a current-limited charge voltage around 14 volts as fully charged its about 13.6v. Connecting 2 SLA together equalises the voltage between them depending on the internal resistances, it charges the lower battery but discharges the upper, unless you have another low-impedance energy source, eg alternator, in which case its best to charge both from that.

The problem with the voltage monitors above is that they tell you nothing about the state of charge of the battery, they tell you the instantaneous voltage without reference to the load on them. The only accurate way to measure SoC is to measure battery volts open circuit after an appropriate resting time which depends on usage, exact chemistry, etc. and could be several hours, or to measure the charge current in and usage current out (so-called 'coulomb counters').

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#### christiannielsen

Joined Jun 30, 2019
337
What do you mean by 'over discharging'? whats the basis of the test?
What sizes are the batteries?
I was thinking if the car battery is discharged to 9 volts etc, then my car won't start. Right now my car battery measures 14.2 volts and it says 44 Ah on it.

I havent decided the size of the second battery yet due to my lack of knowledge about this matter. Could it be 5-7 Ah if only considering what would be preferable for the charging conditions?
The application it is going to power is a GPS that may use very little power in sleep mode and 30 mA when running. But, I dont know yet how often it will sleep and how often it will be running so I don't have a very qualified opinion on the size of the battery.

Anyhow, it's just a hobby projekt and I like the proces.

#### crutschow

Joined Mar 14, 2008
30,105
U1 is upside down.
The cathode side goes to ground (example circuit below):

#### Irving

Joined Jan 30, 2016
2,775
U1 is upside down.
The cathode side goes to ground (example circuit below):
Sorry but Im not seeing the issue, on both mine and your diagram the anode is grounded?

#### Irving

Joined Jan 30, 2016
2,775
Thank you! I am afraid I have to order the TL431 now. I'll get back to you when I've tested it. Will this work properly if the second battery is fully charged (let's say 13.5 volts) and discharged (let's say 9 volts) or do the reference voltage have a minimum voltage?
Yes, the TL431's internal 2.5v reference is independent of supply voltage and that circuit will work fine from 9 to 14v.

9v for an SLA is not discharged, its dead! You should never go below 10v (some say 11v) if you want it to have a reasonable lifespan!

From MK website:

Can continual undercharging harm a Sealed VRLA battery?

In many respects, undercharging is as harmful as overcharging. Keeping a battery in an undercharged condition allows the positive grids to corrode and the plates to shed, dramatically shortening life. Also, an undercharged battery must work harder than a fully charged battery, which contributes to short life as well. An undercharged battery has a greatly reduced capacity. It may easily be inadvertently over discharged and eventually damaged.

Does depth of discharge affect cycle life?

Yes! The harder any battery has to work, the sooner it will fail.
The shallower the average discharge, the longer the life. It’s important to size a battery system to deliver at least twice the energy required, to assure shallow discharges. Follow these tips for the longest life:

• Avoid ultra-deep discharges. The definition of ultra-deep discharge may vary with application and battery type.
• Don’t leave a battery at a low stage of charge for an extended length of time. Charge a discharged battery as soon as possible.
• Don’t cycle a battery at a low state of charge without regularly recharging fully.

Use the highest initial charging current available (up to 30% of the 20-hour capacity per hour) while staying within the proper temperature-compensated voltage range.

#### Irving

Joined Jan 30, 2016
2,775
I was thinking if the car battery is discharged to 9 volts etc, then my car won't start. Right now my car battery measures 14.2 volts and it says 44 Ah on it.

I havent decided the size of the second battery yet due to my lack of knowledge about this matter. Could it be 5-7 Ah if only considering what would be preferable for the charging conditions?
The application it is going to power is a GPS that may use very little power in sleep mode and 30 mA when running. But, I dont know yet how often it will sleep and how often it will be running so I don't have a very qualified opinion on the size of the battery.

Anyhow, it's just a hobby projekt and I like the proces.
9v is far too low. 11v is discharged as far as I'm concerned.

What voltage does your GPS require? What GPS is it?

You need to know your operating requirements before you can determine your power supply provisioning. An alternative battery chemistry may be beneficial.

#### ThePanMan

Joined Mar 13, 2020
430
The issue with what you are proposing is that the second battery will never be properly charged so its operational time will be short and its lifespan compromised - SLA are happiest when fully charged. A SLA battery needs a current-limited charge voltage around 14 volts as fully charged its about 13.6v.
What @Irving said is correct.

Assume you have two identical batteries - same voltage same amp hour rating. Suppose one of them is at 12 volts and the other at 6 volts. The battery at 12 volts will discharge into the battery at 6 volts until the 6 volt battery equals 9 volts and the 12 volt battery equals 9 volts - TOO!

Now, your big 12 volt battery will increase the small 12 volt battery until they equalize in charge. Irving went on to say that the smaller battery will be damaged.

The proper way to charge a 12 volt battery is to charge it at a higher voltage than the nominal voltage rating. A 12 volt Lead Acid car battery is 12.6 volts when in top condition. Over the years of use it will likely fall to 12.2 volts (where my wife's battery is at right now - resting). When the engine runs the alternator and internal regulator boost the voltage to potentially 15 volts. That's on the extreme side of the charging system and more commonly with an older battery you'll see around 14 volts. As the battery state of charge is restored the voltage will drop down to and float at 13.8 volts. That is how a 12 volt car battery is charged. An SLA (Sealed Lead Acid) battery is much the same only different in the form of the electrolyte. The same basic characteristics apply to both a Wet Lead Acid (car battery) and an SLA.

What you're asking for will not work. Not unless you use a Buck/Boost converter. Then your small 12 volt SLA stands a chance of being properly charged. However, penny for penny your car battery will spend more in energy than will go into the smaller SLA battery. It's like spending $10.00 for (guessing)$8.50's worth of charge. It can be done but anyone with a logical mind would say "Why do that? It doesn't pay to do so. Not unless you're a prepper who has a surplus of batteries and want to use more portable sized batteries." Yes, you can do it - but why?

If this is in a vehicle, connect both batteries to the 12 volt system and when the engine is running both batteries will be charged to full. OR if you're going to use a plug-in charger - just hook it up to the SLA.

#### christiannielsen

Joined Jun 30, 2019
337
You have me convinced. Thank you for all input. I will try the battery level circuit though when I get the TL431.

#### ThePanMan

Joined Mar 13, 2020
430
If I had to charge a small battery from larger ones, I'd hope to have access to two 12 volt wet lead acid batteries, put them in series, then construct a current limiting circuit and a voltage regulator so that I can charge the smaller SLA battery to 13.8V float charge. Having two 12V WLA's in series gives you more than enough voltage to fully charge the SLA. 25.2V fully charged WLA's regulated down to 14VDC then current limited to 5A should do just fine. As the battery nears a full charge it will not draw the full amperage available, it will draw what it needs. It may take overnight to charge because in an automotive situation the voltage can go as high as 15 volts. The higher voltage translates into higher amperage to recharge a 12V WLA car battery. As the charge comes up the voltage drops to around 13.8 volts. I don't know if the regulator is a smart regulator or if that's just the way things work out. Charging car batteries has never been something I've bothered much to do or understand. For me - dead battery? Put it on the charger. When sufficiently or fully charged, take it off.

 Just used one of those portable jump starter units that have an SLA battery inside, used for boosting the car battery when needed without the need to find someone willing to jump my car. Used it to pump up some tires on my dolly. Plugged it in to recharge it and the voltage said 14V. From past experiences I know it will drop down to 13.8V while continuing to recharge. Once charging is done it will maintain about 13.2V. If unplugged it should drop down to about 12.6V. It's an older unit so the SLA may drop down to 12.3, maybe a little lower. One thing with SLA's, ALL lead acid batteries, is that you have to keep them charged otherwise they sulfate and become useless. I've had my share of SLA's go bad because I didn't keep them charged. Somewhere I heard the recommendation was once every three months regardless of the voltage reading. You can have 12V and virtually no power because the sulfate has clogged the plates.
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