The Seventy Per Cent Debate
- how vehicle batteries really charge

Years ago I stated that few vehicle batteries get charged much beyond 70% or so - and explained why. This comment was seized on by journalists and eventually appeared on websites worldwide. Over a decade later it still does.

But as many writers know to their cost, there can be chasms between what’s intended and what others read into it or miss. Within weeks, people somehow arrived at totally wrong interpretations. What I thought I had clearly written had morphed into ‘Collyn says that car batteries cannot charge beyond 70%’. Or worse still: ‘charging is terminated at 70%’. This then is a further attempt to show what happens.

The Wanderer’s readers are mostly non-technical so I have no choice but to describe batteries and their charging in substantially non-technical form, using analogies that are only more or less applicable and then to a limited extent.

The article’s intent however is to try to show in as simple a way as feasible what happens to batteries in typical real life situations. This is that:

1. Vehicle starting batteries in normal everyday suburban driving are unlikely to charge much beyond 70%. If charged (via driving) for longer, they are likely to exceed this by a few per cent. If charged long enough (e.g., for days) they may approach or attain 100%. Some may go onto be overcharged.

2. Adding auxiliary batteries changes the above. In such situations, battery makers themselves suggest that 65% charge is rarely exceeded. I am in fact sticking my neck out, and occasionally get admonished, when I suggest 70%. The above however does not apply nearly to the same extent with AGM batteries, nor if more sophisticated charging technology is used. Then, close to or an actual 100% charge is achievable.

If you are totally tossed by technology (or even alliteration) you really do not need to read beyond this. That’s why I’ve put the summary (i.e. 1 and 2 above) up front.

Those who understand electricity, but not necessarily batteries, may care to read a more technical description as well as this article (the heavy stuff accurately describes the charging process - but rarely of that process’s real life implications).

A vehicle’s battery provides energy to start the car, power the clock, maintain radio programs in memory, run the power locking etc. It also energises the starter motor and all else required to start and initially run the engine. But, once the engine is running, the alternator provides all the electrical energy that’s needed most of the time.

Its priority is to replace the small amount used for starting. Once that is achieved the battery acts like a bank, sort of smoothing things out and providing or taking in energy as needed. Mostly though it’s along for the ride until the engine stops.

The 70% charge mentioned above is sufficient to make things work because the associated electrics are designed accordingly. That level is not a problem if exceeded (and, contrary to the now common belief, that can and does happen). There is another reason for limiting the charge and that is that conventional batteries start gassing around 75%. When/if that is exceeded, they need more frequent topping up. So that too was an original reason for limiting charge level in normal usage.

The major reason for the technique’s ongoing use is that it is simple, reliable, cheap and, in normal driving usage, very unlikely to overcharge and possibly cook the battery. It’s like the classic advertising claim for the VW beetle: “It’s simple - but it works!”

Over-charging (i.e., continuing once 100% is achieved) wrecks batteries, but the standard vehicle regulator lacks a system (as such) that prevents this. Instead and usually effectively, overcharging is averted by charging to an acceptable level and then tapering off. Its a bit like limiting road speed by limiting engine power in that it normally restrains speed to a safe level, but if driven down a long enough hill, the vehicle will attain high and possibly dangerous speed. But in any normal usage it won’t.

So for any number of good reasons, vehicle charging systems have a sort of de facto mechanism that enables them to keep a car battery at adequate working levels over a wide range of typical usages, and to a higher but still safe level in all except rare all-but continuous usage. But, if you do drive continuously, the battery may well get cooked.

It’s not that the system cannot charge beyond 70% or so. It can and sometimes does. It’s that in typical usage it is unlikely too do so by much.

As long as a starter battery is over 67% or so, the actual percentage makes little odds. If it’s higher you will be able to crank the motor for longer. But any engine in good order starts in a second or two anyway. If it doesn’t it’s trying to tell you it needs fixing.

How Charging Works:
In the accompanying sketch, the bits on the left represent a vehicle charging system.  This can be seen as device that generates electric current via a device called an alternator. The alternator’s output is controlled by an associated regulator that keeps that current at constant pressure (called voltage).

Current can be visualised as water, voltage as pressure pushing that water. In effect then, the output could be seen as water under constant pressure.

In reality there is a source of electric current that is maintained at constant pressure (called voltage), at a so-called 14.4 volts or so.

As current is drawn, the regulator constantly maintains the pressure (voltage) by adjusting the pump’s (i.e. alternator’s) voltage output. Alternator and regulator thus have different but inter-related roles, but the regulator rules the alternator’s voltage output at all times.

The right hand bit of the drawing represents the battery to be charged. The battery is connected to the charging system (left) by a cable to the charger. That cable has a restriction (explained later) where it enters the battery.

This battery has a level mark at 70% (about 12.6 volts), and another at ‘Full’ (about 12.8 volts and 100% charge).
 
Charging in Typical Usage
Assume the battery is all but flat and thus needs charging.

We turn a (not shown) equivalent of the ignition key. This prepares the alternator and a few other bits for action. Turning that key a bit further starts the car engine (not shown) that drives the alternator. In real life current is generated almost instantly and, urged on by 14.4 volts (pressure) begins to flow through the cable (pipe) and into the battery. I’ll come to the internal restriction later.

The amount of current that can flow is limited by the size and length of that connecting cable. If that cable is too small, current flow rate will be severely limited. To avoid this we need big cables.

As the current (visualised, if wished, as water) continues to flow into the battery, that battery’s level (voltage) rises. But as the now charging battery’s voltage rises, the constantly fixed voltage pushing it finds such pushing increasingly hard to do. It’s like someone pushing a barrow of concrete up an increasingly steep hill. Directly because of this, the rate of charging thus constantly decreases as the charging battery gains that charge (and its voltage increases).

Batteries resist being charged, and that resistance depends on their type and their temperature at the time: charging current then, does not flow easily. It needs to be pushed. Much of the resistance is because the current has to cause electro-chemical reactions and these dislike being hurried, and especially when cold.

There are other and various causes of resistance to charging, but the restriction (shown as a smaller ‘pipe’ where current enters the battery) represents the total resistance to charging, of everything so concerned, that is inside that battery.

What all this amounts to is that pushing current into a battery needs a volt or two more than the battery’s voltage at the time. Increasing that pushing voltage causes the battery to charge faster, but there’s a limit to how far this can go without stressing it. In practice, a nominally 12-volt battery is typically and safely charged in a vehicle at somewhere between 14 and 14.5 volts.

The main thing to grasp is that vehicle systems generate a constant voltage. This brings a battery fairly quickly to about 12.5 volts but from thereon, the charging tapers off. The typical 70% charge results in their being about 12.6 volts across a starter battery some minutes after charging has stopped.

If the engine continues to run beyond that 70% condition, charging continues, but the rate at which it does has by now fallen to only an amp or so. But, given long enough, it can and may fully charge that battery. It may keep on charging after that, and batteries do not like this.

To repeat, the point so often misunderstood is not that a typical vehicle battery cannot charge beyond 70% - it is that in typical driving it is unlikely to. In most suburban driving, batteries tend to have 70% or so charge, but those in cars driven for longer may reach 80% or more. As long as it’s about 70% or over it simply does not matter. It’s pointless discussing it!

Adding auxiliary batteries however changes all this. These batteries may be many times the capacity of the vehicle battery and, in camp, are often discharged to way below 50% charged. Worse, traditional deep-cycle batteries resist charging more strongly than do starter batteries. AGM batteries however have less resistance so they charge faster and deeper from normal charging voltage (and systems).

It is when auxiliary batteries are used that charging is less likely to exceed 70%. And this really does present problems because the makers recommend discharging only to 50% (leaving only 15-20% for actual use). Many users run them lower, but battery life suffers. Fully charging thus makes a huge difference to usable capacity. It may double it. Or even more.

It is the charging voltage that controls the charging rate and, as long as that is maintained (and connecting cables are big enough), it is ultimately the battery’s own resistance to charging that determines that rate. If the alternator is not able to maintain that charging voltage, installing a bigger one will increase the initial rate of charge. But not otherwise because the voltage regulator still maintains that rigid 14.4 or so volts.
 
Better Ways

One but rather crude approach is to install a high voltage regulator. These are available for some vehicles, but you need to discuss this with an auto electrician. This works but can only too readily cook your batteries.

A more preferable approach is to use three-stage’ charging (also used in up market battery chargers and solar regulators). This has a major and significant difference in that as battery voltage rises with charge, the charging voltage increases proportionately. In effect the constant charging voltage is replaced by constant charging current for most of the charge and this really speeds things up. These devices are made for charging auxiliary batteries from the existing car alternator/regulator system; and also as mains voltage chargers.

In the latter application, a constant current 10 amp three-stage charger may fully charge a half-flat 100 amp-hour battery in little over five hours. A conventional charger ’10 amp’ charger is likely to stop at 80% or say - and take at least a full day to get even as far as that. But some are right little so and so’s. They just go for it – and can cook a battery overnight.

(This article was originally published in the Campervan and Motorhome Club of Australia's publication 'The Wanderer'.  It is copyright Collyn Rivers, Caravan & Motorhome Books, Mona Vale NSW, but in the general interest it may be reused as long it is reproduced in full and acknowledged as above.

Collyn Rivers