BATTERY CHARGING

Charging ahead…

Ian Fraser urges us not to take our batteries for granted and offers top tips for charging…

eliable batteries are essential to the

Rconvenience and safety of our aircraft (see LA June 2021 article). While they work, we take them for granted but if they quit, they will do so at the most inappropriate moment, causing maximum inconvenience, sometimes completely wrecking plans and even becoming a safety hazard.

Our aircraft often spend days (or even weeks) doing nothing, then fly for an hour or two and go back to doing nothing. In low winter temperatures, with a hard ‘electricity consuming’ start and with all accessories and gadgets on, those few short flights can be nowhere near long enough to recharge the battery, and the battery’s charge level just goes down and down. This is exactly the wrong treatment for a battery, and it is not surprising that many fail prematurely.

While traditional battery chargers can provide a much-needed boost, they can also cause long-term damage and do nothing to help the longevity of the battery. An they must never be left connected after the battery is charged.

Fortunately, however, technology has moved on and today specialist chargers can be permanently connected to look after batteries without long-term damage, and some even help prevent or warn of impending trouble before it becomes critical. In this article we are looking at the challenge of safeguarding the battery, and how technology has met it.

Battery health

The main threat to battery health is a problem called sulphation in which chemicals from the electrolyte contaminate the electrode plates, gradually isolating them from the electrochemical process. As a battery discharges, lead sulphate builds up on the plates and, as you re-charge, should be ‘reabsorbed’ back into the electrolyte. But the charging process is not 100% efficient and for each charge cycle a little bit of the sulphate is left on the electrode, and it gradually builds up. It is a natural process, part of the battery’s chemistry, and will eventually be the main cause of the failure of the battery.

While sulphation happens as part of the normal process, its rate is governed by the state of the battery. In a well-charged and maintained battery, it happens very slowly, but if the battery is allowed to remain for any length of time in an (even partially) discharged state, it accelerates. A key to battery life and health is to maintain it at full charge – and doing so can double its useful life.

But, if you are not using the battery, it is not going to go flat, is it? Yes, I’m afraid it is. Another natural feature of a lead / acid battery is a characteristic called ‘self-discharge’. Notwithstanding drain from

Top Concorde battery, typical of those fitted to many Permit aircraft.

Above A traditional taper current charger, avoid using if at all possible

any electronics left on – clocks, alarms, cigarette lighter devices etc – even an unconnected healthy lead-acid battery can lose about 5% of its charge in a month, an unhealthy one up to 30%. One of the causes is those sulphates. I sometimes hear it said, ‘...my battery happily starts the aircraft after six months of zero use, so it’s not a problem’. Maybe it does, but it won’t for much longer and it will become a problem prematurely. The damage has already been done to its life expectancy. The worst thing you can do for your battery life is to allow it to become discharged, but the next worst thing you can do is to overcharge it in an attempt to avoid the problem!

Safe and efficient charging

Safe and efficient charging is all about juggling volts and amps. A battery charger, or your alternator, forces electricity into the battery by presenting it with higher voltage than the battery itself is producing. The result is a flow of electricity into the battery which ‘absorbs’ it so that it gradually becomes charged.

A charged lead acid battery has a nominal ‘off load’ voltage of 12.5v, so presenting it any voltage above that will charge it. The higher the voltage, the higher the potential charge current (amps) and thus the higher the rate of charge – at 12.5v the rate of charge will be negligible but at 14v a flat battery may accept a charge rate of 20 amps or more. However, the rate of charge is generally determined by the capability of the charger itself (for example if you use a 5-amp charger that will be its limit).

Once the battery gets toward its normal ‘charged’ state, its rate of ‘absorption’ reduces and it ‘resists’ the charge current but, unless controlled, a traditional charger’s voltage will go up in response, still trying to deliver that high current. In such a situation, another chemical process, electrolysis, takes place altering the electrolyte.

So, if the battery is already charged, excess voltage can permanently damage the battery chemistry, its performance and its physical well-being. This meant that effective battery maintenance used to be an attention-intensive balancing act, but modern technology now provides chargers that can look after the battery automatically, adapting to its condition and state of charge. These are often known as ‘smart’ or ‘float’ chargers.

The smart charger’s task

There are three main stages necessary to efficiently charge and maintain a lead acid battery. The initial stage provides the bulk of the charge, typically at the maximum current available from the charger. The first ‘smart’ thing the charger must do is to stop increasing the charge voltage when it reaches (typically) 14.4v to avoid any damage to the battery, something older or simple chargers do not do.

By maintaining the charge voltage at 14.4v, as the charge state of the battery increases, the charge current will decrease as the battery becomes saturated or fully charged. This is often referred to as the ‘absorption’ stage.

To continue charging a full battery at this voltage will damage it, so the next ‘smart’ challenge is to detect the reduction in the charge current and reduce the charge voltage to a lower level (typically 13v-13.4v). This is intended to maintain the battery at 100% charge (i.e. to counteract its tendency to self-discharge) without damaging it. This last stage is the ‘float charge’ and is critical to maintaining the long-term health of the battery.

The battery charger marketplace

‘Smart’ is just a marketing term, it doesn’t have a TSO or other standard behind it to give it any credibility – a primitive charger decorated with a bow around it could also be referred to as a ‘smart’ charger! Beware the hype, it’s what the charger does and how it does it that is important.

Above Pictured is a conventional charger hitting a charged battery with 16.5 volts. Not a healthy situation.

A cheap entry level charger (known as a taper current charger) is just a transformer and diode, with no control electronics. For much of my life that is all a car battery charger was, and there are still many of them around hangars and garages. They hit the battery with 15v or more, relying on the battery itself to control the charge, and once the battery has finished absorbing charge the voltage can go up to 18v or more. While they will charge a battery, they can inflict severe over-voltage damage in quite a short time, so avoid them unless it’s an emergency and never leave them connected after the battery is charged.

DC voltage regulators (constant voltage chargers) are sometimes sold as ‘permanent connect chargers’. They are generally quite cheap, but how effective and safe (to the battery) they are depends on how their single ‘regulated’ voltage matches the battery, its task and use. Their intended use is for very specific equipment / battery configuration or powering Christmas lights, and not as general-purpose chargers.

The best solution to battery maintenance is the three-stage charger, as described above, which adapts to the specific task being asked of it; whether it be recharging a flat battery or looking after it during the unflyable months of winter. It is a mixture of constant current and constant voltage. The key feature to look for is the three smart stages and most reputable manufacturers will publish details of them in their specification. Avoid it if it is not there.

Does size (amps) matter?

Yes, it does. The maximum charge rate for your lead acid battery should be no more than 25% of its capacity (amp hour rate) above which damage may occur. The ideal level is no more than 10% of the capacity. Battery charging is a relatively efficient process (80%+) so, as long as your battery is healthy, you can easily calculate how long it might take you to recover a flattened battery. For example, my RV-6’s old 22ah battery should be completely recharged from flat in five hours by a 5-amp charger, and that is the quickest I can safely do it. If I don’t need a quick charge, 2 amps would be better for the battery. If all you are doing is preserving the battery, then a small motorbike charger will suffice (typically less than 1 amp). That might take 24 hours to recover my test case battery, but why does that matter? It was one of these that successfully maintained my RV-6’s battery for more than 12 years (the previous one lasted six).

Some smart chargers have two or more modes, a motorcycle mode (that just means ‘it is for a small battery’ and limits the bulk charge current accordingly) and higher charge rates if you need a quicker result, it’s cold or you are running aircraft electrics in the hangar for any length of time.

What should I consider?

I am not attempting to undertake a product review here, but will mention two companies whose products certainly dominate the hangars which I visit and that have a good track record for maintaining aircraft batteries well – CTEK and Optimate. They have also both been helpful in the preparation of this article.

Both companies’ products offer the basic stages that I have mentioned, however there are various other smart things that they do depending on the particular model. These additional features include testing the battery’s ability to take a charge, de-sulphation or reconditioning functions (that work as long as it has not got too bad), through to the ‘start current’ tests, which I mentioned in my June article.

In particular, the use of pulsed charging as opposed to constant voltage or current methods has opened the door to squeezing that last 1% into the battery without damage, thus reducing sulphation even more.

As with any market, you get what you pay for but even the basic small chargers offered by both at £30-£40 are more than up to maintaining our aircraft batteries.

Most existing lead acid chargers, whether smart or not, are incompatible with lithium batteries but as they become more commonplace, the very latest generation of chargers are becoming compatible with both types.

Solar charging

What about those of us who don’t have mains electricity where we park? Solar cell powered chargers have been around for some time, but to date have not enjoyed a very good reputation. At the low levels of energy available in a UK winter, they just didn’t reliably operate a battery charger. To achieve any form of reliable charging requires a large solar array, but unless regulated by an effective three stage charger, this could damage a battery on a sunny day. Thus, such devices have not come into general use. Top The CTEK small battery charger is ideal for battery preservation.

Above A more advanced CTEK charger / tester.

Right Optimate’s special charger for lithium batteries is recommended by some manufacturers.

However, Optimate has a solution. I described my RG25 challenge to them and they recommended and lent me a 20w solar TM522-D2TK to try. It is aimed at the motorbike ‘wintering’ market, so it is suitable for the small batteries typical of our aircraft. Its solar array is only 430 x 340mm and mounted quite easily on its rubber suction cups in the RV’s canopy. It could also easily rest on the panel of many Permit aircraft or, of course, on a mast next to the hangar.

Its minuscule controller capitalises on electronic techniques to optimise the efficiency of the solar cells and combines this with the three stage smart charger functions, using pulses rather than DC to achieve charging with the very low energy (I measured as low as 2 watts) available on dull or wet days.

I tested it on my old battery, which I ran down to about 50% charge. What I needed to test solar charging was a representative period of dull weather, and this June didn’t disappoint – in Somerset it delivered four of the worst days for some time. It took Above left A solar array mounted remotely on a non-electric supply hangar.

Above right The very compact controller for the Optimate solar system handles lead acid and lithium batteries. Left Optimate’s solar array mounted in the windscreen of Ian’s RV-6.

a couple of days to get the battery back to 90%, and a further day before it indicated float mode (fully charged) reliably. In the shorter winter days this could take longer, but under normal winter aircraft use that would be fine. I was quite impressed with the results; it would certainly maintain my RV-6’s battery over winter, even when mounted in the windscreen.

Battery chargers are one of those tools you use without much thought, often with short notice because something has gone wrong, but today full-time use can contribute much to the reliability of the flying experience. Used wisely they will pay for themselves in consistent starting, reliability and battery life, however if used without caution they can do more damage than they save.

Note that while the principles for lithium are similar, the detailed process and values are different and there is some scientific conflict as to whether the float charge stage is good or even necessary for lithium batteries. ■