A 12V Series Mode Solar Charging Regulator For A 12V Lead Acid Battery

Introduction

The Series Solar 12V Battery Charger Regulator In Use

Click to enlarge any image.



Back in the heady days of September 2021, in the UK we were out of strict COVID restrictions, toddling along OK, and I decided to venture North and stay with a good chum at a small country cabin belonging to my brother. This is equivalent to posh camping. It's very nice, has everything that you need, and even a small old 12V car battery which was charge maintained by being connected to a 1 Watt solar panel hung in the window, with additional occasional charges on the bench at home. Here's what one of those small panels looks like.

Picture Of The 1 Watt "Battery Saver"

Henry's Solar Panel Top Tip

Can you see the horizontal cell divisions on this amorphous silicon solar panel? If you somehow cover up half of this panel vertically, you'll still get some output. If you cover up just one of the individual cells horizontally, you'll get nothing. That dark cell will block the entire output. This is relevant if you're hanging such a panel in a cabin window on a piece of string. The string has to be the right length to allow all of the panel to be exposed.

These are better than nothing. If your car is sat on the drive for two weeks at a time, or if you just want to run some small LED cabin lights from an old battery, and you can get some days when it is in full sunlight, no problem. I've seen 40mA coming out of these while charging a 12V battery in full sunlight in the UK. That might be 80mA in the Sahara at mid-day. They are advertised as 1 Watt. So as P=VI, 1/12 = 0.083 Amps. You can put 40mA into a sealed lead acid car battery for 8 hours a day all year and be sure of never over charging. However, as I was staying for free, I thought that I could make something that was a bit better. If you want to charge your mobile telephone or laptop, you're going to need a solar panel system that provides more current. That also means that you have to avoid over charging that old car battery. If you have a very nice caravan storage battery, the same thing applies.

Lead acid battery charging is fascinating. For that, you can search online for "Battery University," or anyone who is likely to sell you a caravan storage battery.

A key requirement with any battery is that you don't short it out. Automotive car starting batteries or caravan storage batteries can produce an enormous current. Short out a car battery at the end of some wiring and those wires will instantly glow orange-hot, ignite the PVC insulation and set fire to the whole place. Installations left unattended need to pay particular attention to this threat. Big Brother was particularly keen to emphasise this issue, as well he should. I set about to make something with sufficient built-in safety that could get more solar charge into any given 12V battery, given an old Craplins "12 Watt" 12V amorphous solar panel that I'd bought 12 years earlier. There's no point having it sitting in the dark underneath the stairs, eh?

If you have a boat, you might have one of those small wind turbines or solar panels. In that case, what you normally do is connect all the panels or turbine directly to the battery, but also have a shunt regulator connected to the battery. The shunt regulator dissipates the unwanted power as heat when the maximum battery voltage is reached. This is done for maximum efficiency between the source and the battery. The shunt regulators can get rather warm dissipating the excess power. Here we are not looking for maximum efficiency. What we want this time is to compromise some small amount of efficiency for a regulator powered by a medium to small small solar panel that you are happy to leave alone for years charging a 12 Volt sealed lead acid battery. You can use a semi-sealed low maintenance lead acid if you want, or even an open cell type.

Battery University mentioned above will tell you that there is a temperature coefficient involved in the cell voltage for lead acid batteries. Ideally might want to track that when charging. 13.80V is the standard float charge for six lead acid cells at 25ºC. Float charge voltage is the voltage where you can sit at for a long time without damaging a sealed battery. The temperature coefficient is -1.8mV/ºC for each cell. It's not a tremendous amount. Adding temperature compensation would be a nice bit of finessing, but I've not included it in this project. If you're going for the ultimate solution, there are ways to do it, but it gets awkward if you want to measure the actual temperature of the battery. You won't see temperature compensation on any but the most fancy systems, certainly not a lead acid car battery bench charger or a shunt regulator. Submarines, yes, but then the modern ones have air bubble blowers for agitating the acid as well, and then we're getting silly. You don't need it.

13.8V Solar Series Regulator For Charging 12V Lead Acid Batteries Schematic solar_series01-01.sch PDF



Input from the 12V solar panel comes in on a female XLR socket on the left. There's no great need for any fusing or protection here. Shorting out an 18 Watt solar panel isn't going to do any harm to anyone. A "12V" solar panel usually refers to one which is capable of charging a 12V lead acid battery at the rated panel wattage when connected up directly, so the open circuit voltage can be about 18V. This connects into an LM1086 low-dropout regulator. C1 and C2 provide high frequency stabilization for the regulator. The regulator is set up by R1, R2, and R3 to produce about 14.3V at the pin 2 output. A hefty low voltage drop power Schottky diode D1 follows, then a 3 Amp panel meter. The  2 Amp fuse is the last thing before the male XLR output connector to ensure that any failures internal to the unit will not cause a short circuit on the battery. A centre-off DPDT switch allows a 15V panel meter to be temporarily switched between the input and output voltages. The centre-off switch is a biassed type that springs back into the middle "off" position when released. In this way it ensures that you can never leave the voltmeter connected.

How do you set the voltage on the regulator? We're looking to float charge a lead acid battery at exactly 13.8V at 25 Centigrade temperature. We'll call it 13.8V and ignore the temperature coefficient. You could remove D1 and set up the resistors so that U1 would produce exactly 13.8V, and all would be fine. Unfortunately, in the dark, the battery would be discharging via the resistor network and the regulator. D1 is a reverse discharge blocking diode that stops this. But D1 has a forward voltage drop of about 0.5V at 1 Amp, and it varies with the current. What are we to do?

A car battery of any sort will take a certain amount of charge current and not be damaged, no matter how fully charged it is. Let's say 50mA. 50mA is quite low. So you apply 16V input to this charger box and measure the output current, and adjust the applied load to 13.8V. (Or wait until the battery connected reaches 13.8V)  You then select-on-test R3 so that you see less than the arbitrary 50mA current that you first thought of. In practice, it's not as critical as all that. In full sun, U1 pin 2 ends up at about 14.3V, the battery voltage climbs up towards 13.8V, the current reduces and by that time the Vf of D1 is higher than 0.5V anyway, and you'll never over charge your battery. For once, the gods of silicon and resistance are helping us. You just choose an appropriate compromise depending on the size of the battery involved. The values shown are pretty good for a wide range of possibilities.

You can use a preset variable resistor for R3 if you like, but I hate presets. The wipers go open-circuit and they offer too many options for fiddling by yourself or by others. Make your mind up, test, and fit a fixed resistor. You'll never regret it.

The regulator is bolted to the substantial diecast aluminium case. This provides more than enough heat dissipation even when charging a discharged battery in full sunlight. The mounting tab is not the ground pin, so this regulator requires a mica washer, an insulating washer, and the usual horrible white heatsink grease. A nice feature of the series regulator is that the device only gets warm when you're charging. That will usually be the case when you're in attendance to keep and eye on things. This is in contrast to shunt regulators. They get hot when input power is available and the battery is full. That will typically be when you're not there using power.

The output lead connecting to the battery has a 2 Amp fuse really close to the positive connecting clip. This ensures maximum protection against possible short circuits in that lead. The regulator box itself already has another 2 Amp fuse, and could be considered to be redundant. It does however provide secondary protection against someone shorting out the external fuse with tin-foil, which idiots will occasionally do. Use the modern automotive spade terminal type of in-line fuseholder. The old tubular fuseholders with the internal springs going over the connection wires make terrible connections.

The voltage regulator chip has on board over current protection which kicks in at about 1.5 Amps. This protects against being connected to an over sized solar panel. The regulator chip also has thermal cut out protection. This protects against the unlikely chance of it getting too hot which could occur if, for instance, the bolt holding it onto the case became loose. In the picture at the top of the page you can see the unit in use charging at about 400mA. Having the meters available is not strictly necessary. Moving coil types are not particularly accurate. They are still very handy for seeing that everything is working. It's very gratifying when the sun is shining and you see half an Amp going in at 13 Volts.

Pictures Of The 13.8V Solar Lead Acid Battery Charging Regulator Under Construction

Internal Metalwork



I hate metalwork, but it has to be done.

Solar Series Regulator Top Angle View



Solar Series Regulator Top View



12V Solar Panel Input Socket On Female XLR



Solar Battery Regulator Output Male XLR And Fuseholder



Three pin XLR audio connectors are not specifically intended for DC power use, but they can't be connected the wrong way round and have plenty of current handling capability. They are good quality items if you avoid the fakes. You see them used for the charging input on some mobility scooters.

Solar Series Charger Inside Wiring



Note the use of direct soldered connections to the circuit board and doubling up of some of the higher current wiring.

Solar Series Charger Circuit Board



Solar Series Regulator In Use Showing Battery Voltage



That's 0.4 Amps at about 13.2V. The voltmeter is somewhat enthusiastic and reads a bit high.

Solar Series Regulator In Use Showing Panel Voltage



That's a 2 Volt drop through the system. At 0.4 Amps we're wasting about 0.8 Watts as heat in the regulator and diode. This waste is what shunt regulators try to avoid by connecting the panel directly to the battery. We'd rather have the additional inherent safety of a series regulator in this particular case.

Solar Series Regulator In Use Overview



That's a packet of spare 20mm 2 Amp fuses sellotaped to the outside of the box which should be completely unnecessary. The tubular fusholder shown has been replaced with a spade type.

12V Solar Powered Car Battery Charge Regulator, The Simple Version

Close observers will note that in the schematic diagram, there's a simpler version without any panel meters. Here it is. I use the same type of 12 Watt panel connected to my car battery via the simple regulator. The panel is the one on the right in this picture. It has been much repaired over the years. The urban foxes love to chew through the cable which leads round to the front of the house.

Solar Panel Used For Car Battery Maintenance At Home



Simple Version Of Solar Series Regulator In Use



Here's the simple version in use, showing that important short connection to the battery terminal before the safety fuse. It's difficult to see how you could short that out. The use of circular charger connectors here is a compromise. They are not as good connectors as the XLRs, but the panel input plug will pull out quite easily if you accidentally reverse the car off the drive without disconnecting the charger box first. Yes, I have tested this.

Simple Solar Series Regulator Panel Input Socket



Simple Solar Series Regulator Battery Output Socket



The two heatsinks on the top have proved to be unnecessary. The diecast box alone can easily dissipate the low number of Watts that could be encountered under worst case conditions.

Simple Solar Series Regulator And Battery Connection Lead



Simple Solar Series Regulator Insides



Here you can see that all the components are self supported on the input output sockets and a grounded solder tag.

The Series Lead Acid Battery Solar Charge Controller From A 2026 Perspective

And Finally...

Talk of fuses in 12V battery systems reminds me of some CB radio japes in the past. As a teenager I would regularly be presented with the CB radios of local users where they had been connected up to the 12V DC power the wrong way round. What happens when you do that? The first time, if you're lucky, the 2 Amp fuse in the power leads blows, you replace it, and away you go. These were not the kinds of sets that I encountered. I always saw them when that external fuse had been removed, or replaced with a piece of foil from the inside of a pack of 20 B&H Gold. Inside the radio, the 1 Amp reverse polarity protection diode would be vaporised. The TDA7205 audio amplifier chip would have a piece of plastic blown off the front of the chip. The electrolytic capacitor across the main rail would have exploded, sending bits of fluff and foul smelling smoke all over the insides. The RF PA transistor would often be dead. The LED channel display was occasionally broken which could usually not be replaced. The 8 Volt regulator for the transceiver sensitive bits would often have gone to silicon heaven as well. Fused PCB tracks were what often avoided complete destruction.

I'd replace all these parts, but with a hefty 5 Amp reverse protection diode and an internal 2 Amp fuse secreted in some insulating tape. I'd make very little money on this, and you'd never quite know how big the parts bill was going to be. The radio thus fixed, I would hand it back to the user with a stern warning to connect it up the right way round. Would I replace the missing external in-line fuse? No. The radio is fixed.

A week later, or maybe the next day, the set would arrive back dead.

"What did you do?" I would ask.
"I connected it up the wrong way round again." Would be the inevitable answer.
"Oh dear. Same price as last time then?"
"Yup."

Replace the internal 2 Amp fuse, wait a week, hand back the fixed radio, and that's where a bit more cash was made. That's not dishonest. That covers the costs of the harder initial repairs where the customer is going to balk at the price of the parts.

I was once riding with a CB radio chum in his bronze 1976 Hillman Avenger with the usual black peeling vinyl roof. "Duracell," for that was his CB radio handle, had fixed up his CB radio in the car. I asked him which circuit he'd connected it into at the fusebox. "Oh, I drilled a wire though the bulkhead and connected it straight to the battery. I advised him that this was a sub-optimal thing to do, suggesting that it was only a matter of time before the PVC insulation wore through on the metal edges of the bulkhead hole and set his car on fire.
"Oh, it'll be reet," He said.
Ten days later it was a smoking wreck outside his house.

Henry's general email address:

Navigate Up