How to Charge 12Volt Batteries
The battery is in many ways the heart of any 12Volt system, and as you’ll see in the article about Batteries: their quirks and characteristics the lead-acid battery is by far the most popular one at this stage, so that’s the only type I’m going to describe here. If you have a Lithium-based battery then the supplier should be able to point you in the right direction, and Wikipedia has a pretty good fact-based section on these batteries too.
Whenever we charge a battery there are two basic modes that the charger can choose from:
In constant-current mode we use electronic trickery to limit the current to the maximum that the charger can deliver. For deep-cycle batteries, to prevent long-term damage we want to keep the charging current at about 10-15% of the battery capacity – so for a 100Ah battery we’d keep the current at 10-15 Amps. If we are in a hurry then the charging current can be stretched to 20% but that shouldn’t be done too often.
In constant-voltage mode the voltage is kept steady at a certain level, and this voltage will be different for each stage of the charging process. Typically the constant-voltage mode follows after the constant-current mode and this is when the battery is close to fully charged.
Constant-current is often referred to as the boost stage, followed by the absorption stage which is constant-voltage at 14.something volts to allow the battery to gradually reach 100%. Once the battery is full the charger drops the voltage back to a constant 13.something volts and it’s then in float mode.
There are a range of different chargers out there, from 240 Volt chargers to solar chargers, and then those that use the power of the vehicle’s alternator. All of them will basically do the same job, and will automatically step from one mode to the next during the charging process.
One of the best ways to charge our 12Volt batteries is actually using a solar panel and regulator.
In a solar system the regulator is the part that controls the multi-stage charging process which includes both constant-current and constant-voltage modes. A solar panel is inherently current-limited, so as long as the panel is not over-sized for our battery, we can’t damage the battery by charging it too fast. This makes solar a really good way of charging 12Volt batteries.
And the relatively simple PWM regulator has one of the best charging regimes for batteries, so solar is a really easy way to get it right. A big if is of course the quality of the regulator – look for one that is either made in Australia (and I stress made) or a well-known brand that is stocked by a number of retailers in Australia. A rough guide is the price – you’ll want to pay more than $100 for a good PWM regulator over 5 Amps, and over $300 if you are going for a 20Amp one with digital readouts and so on.
In Australia MPPT regulators don’t really make sense – they cost more but given our hot summers they can’t give us the same benefits that they do in cold climates like Europe and North America. For more on this, have a look at Solar Panels and Regulators.
Maintenance Solar Charging:
If I just want to keep my batteries from going flat while the ‘van or boat or tractor is idle in the off-season, then a 10 to 30 watt panel should do it. If there are parasitic currents, like alarm systems or radio-stereos keeping selections in memory, then it’s probably a good idea to go a bit bigger. And despite the campfire stories you may hear, you do need a regulator – it can be a small PWM regulator, but without it you’ll almost certainly overcharge your battery and have to replace it.
This is for charging when a 240 Volt supply is available. This can be at a caravan park, or when there’s 240V available from a generator, or simply when the ‘van is parked up at home. The thing to look for here is a smart-charger – sometimes called a multi-stage charger. These ones will step through multiple charging stages automatically and most importantly, can be left connected to the battery for days and weeks at a time. This ability (or not) to remain on for long periods might be in the small print of the instruction manual, so look very carefully.
Battery chargers come in various sizes, and are rated according to the maximum current that they can supply, from under 1 Amp to 25 Amps and beyond. For a given brand of smart-charger they will all effectively do the same thing – charge our batteries – but the higher current charger will just do it faster. So as we go up in current rating we are basically buying time.
If we only want to maintain a battery that sits in the shed most of its life, then a small one will do – there’s plenty of time. But if we have a mobile workshop in the back of a van that returns in the evening and needs to get its battery up quickly by the next morning, then we will choose a larger current charger. Just remember to keep the current under the 10-15% maximum to avoid long-term damage to our deep-cycle batteries. (see also A Guide to 12Volt Batteries).
Emergency Charging from a Generator’s 12Volt outlet
Some generators have a 12Volt outlet for charging 12Volt batteries – usually with a T-shaped socket of some sort. You’ll see we’ve called this Emergency charging – for a number of good reasons.
Firstly, this 12Volt outlet is unregulated. So unlike a smart-charger, it will keep putting as much power as it can into your battery, no matter whether it’s empty or full. This is fine if we have a flat battery and just want to get it charged so we can start the engine – in other words in an emergency. However if our battery is full, this outlet has no smarts to it, so it will simply boil the battery into oblivion – not good.
Secondly, the 12V DC outlet usually has a maximum current of 5-10 Amps, so it’s not going to be as fast as a 15 Amp smart-charger for instance. But there again, if we don’t have a smart-charger available, then we can work out more-or-less how it’s going as it charges, by monitoring the battery voltage. (see table at end of a Guide to Batteries). But don’t go on a little hike while it charges – you might come back to find a ruined battery – a tough thing to explain those who were looking forward to a nice full battery that night…
So the short version is, the 12Volt outlet on Generators is for Emergency charging only.
Charging from the car’s Alternator:
And then there are chargers that make use of the power from the alternator – this makes it possible to charge up the batteries while I’m travelling along. There are two types of in-vehicle charger, either a simple Automatic Charging Relay, also called a Voltage Sensitive Relay, or the more sophisticated DC-DC Charger. Both of them take their power from the starting battery – and any power we take from there gets replaced by the alternator.
It’s probably also a good time to point out that using the starting battery to run fridges, lights and other 12Volt stuff is not a good idea – ask me, I’ve tried it, and it ends in tears. The starting battery is for starting, and when you’re in the middle of nowhere it’s a really good idea to make sure you have a full starting battery. So we need to invest in a second battery – it’s worth it, believe me.
So, to those chargers that work while we’re travelling along.
Automatic Charging Relay (ACR):
As you’d guess from the name, the ACR is simply a relay that connects our starting battery to the second battery, also called the auxiliary battery or the house battery. The ACR waits until the starting battery is charged before connecting the second battery – that way the starting battery has priority – all good. Importantly it also disconnects as soon as the alternator stops charging and the voltage drops – so the starting battery is left alone so it can do its job properly.
Wonderfully simple and cost-effective too, the ACR – until the arrival of the new-fangled “smart-alternator” – also called a variable-voltage alternator. Around 2010 alternators “grew a brain” and after charging the starting battery they drop back their voltage to a maintenance level. This increases fuel economy, and drops emissions, so everyone’s happy, right? Well everyone except the second battery – at the maintenance voltage there’s now very little left to charge it – bugger! So if you have a trusty old ute from 2005 you’re in luck – the ACR will do the trick! If not, keep reading, it’s the DC-DC charger for you…
So, these fancy alternators have dropped the voltage – what to do? Simple really, just boost it back up again – and that’s basically what the DC-DC chargers do. Naturally this involves some electronic trickery and therefore some extra cost, but the good news is that it can be done – we can still charge that second battery using the alternator’s power.
A word of caution – the electronics in a DC-DC is pretty hi-tech stuff and not that easy to get absolutely right. Obviously all those being sold get it right to a large extent but the biggest problem is heat generated by the device itself. The most successful DC-DC chargers are those that are basically one big heatsink, and there’s a really good one that originates from my home state of Western Australia. Even when mounted in the engine-bay with temperatures well over 50°C they still deliver their full 25 Amps of current – I’ve seen it with my own two eyes…
Most of the others that I’ve seen on test will work ok for the first half-hour or so, then the heat builds up, they get hot, and the internal electronics current-limits back to protect itself. All very good for the DC-DC charger but it obviously means that you’re getting less output than you paid for, so it takes longer to charge that second battery.
When choosing a DC-DC charger we can be tempted to go for the biggest available because that will clearly charge faster. Sure, just remember the 10-15% rule for deep-cycle batteries though. So if you have a DC-DC charger rated at 25 Amps then that would be good for batteries around 150Ah and more – smaller than that and you’ll be playing with the battery’s life expectancy.
So that’s about it for charging batteries. If you need to know more about the batteries themselves, head over to the article on Batteries: their quirks and characteristics.
 Instead of saying 10% of the capacity, some datasheets write this as 0.1C (where C is the battery capacity) – same thing.
 One of the parameters listed on the back of a solar panel is short-circuit current (Isc) so this is the maximum current that a PWM regulator can ever deliver to the battery. If there are multiple panels, just add the currents.
 A relay is just an electrically-operated switch, usually with a high current capability.