This article discusses the two main types of battery used in campervans and motorhomes: starter batteries, and deep cycle batteries. Both work in a basically similar way. Energy is stored within them as a result of an electro-chemical reaction between lead plates and a water/acid mix (called the electrolyte).

Charging is effected by imposing a voltage across the battery that is greater than the voltage 'within' that battery. The greater that voltage difference the quicker and deeper the battery will charge. When battery voltage reaches the charging voltage, charging ceases.

A starter battery delivers a very heavy current (up to 400 amps). It does this (hopefully) for only a few seconds. This discharges it by a mere 2%-3% and that small amount of energy is replaced within a minute or two of the engine starting. In practice, starter batteries spend most of their life somewhere between 65% and 70% of full charge. Lead/calcium batteries may be charged a bit higher.

Providing such heavy current requires the lead plates to present a large surface area to the electrolyte. To enable this, starter batteries have a large number of thin plates, but whilst this form of construction allows heavy currents to flow for a few seconds, it will not withstand more than a few extended discharges. Flatten most starter batteries half a dozen times and they are dead

Deep Cycle Batteries

A deep cycle battery differs from a starter battery in that it is intended to operate consistently over a much large part of its capacity. It cannot however supply remotely as heavy a current, in fact it will be damaged if discharged regularly at greater than 25% or so of its amp/hour capacity. Despite their name, deep cycle batteries cannot consistently provide remotely the energy that the label on the side may lead you to believe. Without adequate solar or 'smart' battery charging a deep cycle battery is unlikely to exceed 70% of nominal charge. So there's 30% lost up front. Worse, battery makers advise not to discharge below 50%. As that leaves only 20% for actual use almost everyone ignores the battery makers' advice and many people routinely run them down until the lights go dim (about 80% discharged).

If treated as above, the best deep cycle battery yet made is good for 80-100 charge/discharge cycles. If run down to about 30% remaining charge they are good for150-200 cycles. Doing as the makers advise gives you 500-1000 cycles. Sadly, magazine article after another (and advertisers who should know better) routinely claim a fridge that (for example) draws 5 amps will routinely run for 20 hours on a 100 amp/hour battery. You'll see pigs flying in formation (whilst singing arias from Tosca) before a 100 amp/hour battery can do that! The reality is that 'deep cycle' batteries would better be described as 'less-shallow cycle' batteries.

How Low Should I Discharge?

Because the amount of air shifted by a cooling fan is proportional to the cube of its rotation, low voltage doesn't do a great deal for fan-cooled motors, but otherwise running
the battery way down is unlikely to do any harm. Most fridges have a voltagesensing cutout that disconnects the incoming power if it drops (typically) below 11.4
or so volts. This is usually promoted as for protecting the battery, but its main job is to protect the fridge motor from overheating. Discharge level is essentially a trade-off between convenience and your bank account, but unless you keep the discharge to 65% (i.e. 35% left) you are better off buying the cheaper traction batteries. These, and true deep cycle batteries are likely to last about as long. But please do not take the above as a recommendation to discharge batteries below 50%. I'm simply explaining what happens if you do.

A far better way is to have a charging setup (which will generally require solar) such that the batteries are routinely charged close to 100% and discharged overnight by a probable 15-20%. This way the batteries stay much of the time around 85%-90% fully charged - and last forever. That's how my OKA works and its last set of deep cycle batteries lasted over seven years. (Just how to do this and why, is explained in 'Solar That Really Works!' - Motorhome Edition (obtainable for $37 from the CMCA Head Office).

Microwave Ovens - A Trap

Be aware that an '800-watt' microwave oven may draw, via a 12-volt inverter, close to 150 amps. Such heavy current will damage any deep cycle battery bank of less than 350-400 amp/hours. Campervans and small motorhomes commonly have microwave ovens, yet may have a battery of only 150 amp/hr. If you use that microwave away from mains power, a marine battery is a better bet. This is an exceptional recommendation however as a marine battery's capability, of delivering light starter current yet retaining some deep cycle characteristic, is of no benefit in normal use.Knowing What's Left In a deep cycle battery, especially when cold, the 'charge' held on the lead plates massively leads what's happening in the electrolyte. And vice versa. This is because the electro-chemical reactions are very slow. Because of this, any measurement of voltage taken within 12 hours of charging or discharging is all but meaningless. All you are measuring is the surface voltage of the plates. Hence, a substantially 'flat' battery will present as close to fully charged (>12.6 volts) for a long time after being fast charged for a minute or two. And an almost fully charged battery may present as near enough to 'flat' (<11.8 volts) after driving a microwave oven for a few minutes.

Despite this any camping site will have people 'checking' batteries during the day and making hugely incorrect assumptions on the basis of what they find. Even doing this first thing in the morning, with absolutely everything having been turned off overnight, may still result in 30% or more error. Further, one needs a very good meter to have the accuracy and resolution to measure the critical 0.2 -0.3 volt in 12.7 or so volts. A hydrometer reading is better - but the battery still needs to have rested. Take note only of the hydrometer's specific gravity reading, NOT of the coloured bands and indications thereon. These markings usually relate to starter batteries and indicate CHARGED at 1.280 SG or more. A deep cycle battery is close to fully charged at an SG of 1.250 and is likely to be high as it will go by 1.260. Also, as ambient temperatures rise, batteries charge at lower specific gravities and voltages.

Measuring What's Left

As a very rough guide, a well-rested deep-cycle battery that shows 12.25 volts off-load is about 50% charged. Full charge is likely to be 12.6-12.8 volts but the slightest load may pull that down to 12.55 or lower. Table 1 gives a rough guide, but only after batteries have rested for at least 12-hours. The only really effective way of knowing remaining charge is to measure what goes in and what comes out and deduct charging losses. What's left is more or less what you've got. It's actually a bit more complex than that as the rate of charge/discharge affects the result. This was thoroughly explained, via Peukert's Equation, back in 1897 (email me if you'd like a copy). Almost every good solar regulator costing over $300 or so shows what comes in and what goes out, but does not necessarily include the Peukert correction (although the Mastervolt unit does). Stand-alone energy monitors are also available but if you are going to use solar it's far cheaper to use the functions provided by the more upmarket solar regulators.

Sealed Batteries

Traditionally, the term 'sealed battery' described gel cells and, in recent years, AGM (Absorbed Glass Mat) batteries. These batteries are heavier, bulkier and costlier than conventional lead acid batteries. They charge close to 100% via standard alternators and may be discharged deeper with far less self-damage. They thus provide much closer to 100% of their nominal capacity and this compensates substantially for their greater weight, bulk and cost.

In recent times the term 'sealed' battery has also become a synonym for 'low maintenance battery'. These batteries use a small proportion of calcium, and other things, in their plates to reduce gassing and water consumption. That, plus a larger reservoir above the plates, enables them to be permanently sealed. This seems a good idea for starter batteries (which have an intended life of only two/three years) but I am not yet convinced of their suitability in deep cycle form - where a well-maintained and correctly used conventional battery can last five to seven years - unless you prefer to swap battery longevity for freedom from checking the electrolyte level every eight-twelve weeks.

PERCENTAGE
CHARGE VOLTAGE SG
(<25 DEGREES) SG
(>25 DEGREES)
100% 12.75 1.250 1.240
90% 12.65 1.235 1.225
80% 12.55 1.220 1.210
70% 12.45 1.205 1.195
60% 12.35 1.190 1.180
50% 12.25 1.175 1.165
40% 12.10 1.160 1.150
30% 11.95 1.145 1.135
20% 11.85 1.130 1.120
10% 11.75 1.115 1.105
ZILCH 11.65 1.100 1.090

TABLE 1

Approximate voltages and specific gravities of deep-cycle batteries (rested for at least 12 hours).
Copyright 2003 Caravan and Motorhome Books, Broome WA.

Charging Batteries

A vehicle charging system is deliberately designed to drastically cut back charging at 70% of full charge. The only really safe way to approach 100% charge (90% is a realistic target) is via adequate capacity solar modules and solar regulator; or via a 'smart' charger - also increasing known as a three-stage charger. These latter chargers are not cheap - they start at $300 or so - but so far most of their vendors seem to have overlooked that a 10-amp smart charger will charge a lead acid battery as fast as, and deeper, than any chain-store 20-amp charger. Smart chargers may be left permanently 'on'. If this is done the batteries will last many times longer. Thus, despite their cost, smart chargers save you money in the medium-long term. Like most things in life (except yoga and CMCA membership) with batteries one tends to get back less than you put in - about 15% of the charge/discharge cycle is lost in heat. This needs to be remembered, but not always is, when installing solar.

Maintenance

Keep the terminals clean externally (a tablespoon of bicarbonate of soda in a bucket of water works like a charm). Once a year disconnect the terminals, clean them until shiny on their contacting surfaces. After reconnecting, coat with Vaseline or battery protection goo. Check water levels at least every eight-ten weeks. A correctly charging battery should use some water. About one centimetre every ten weeks is normal in temperate climates. If less, the batteries are probably being undercharged. If much more, and unless you are in a very hot area, they are possibly being overcharged. Avoid Christmas trees of cables hung off battery terminals. Instead, install one or more common power posts, and take a single heavy cable from there to the battery terminal. There's a huge amount more one can write about batteries (I know this because I already have in Motorhome Electrics!) but the above gives a run down on the more important aspects of their choice, care and feeding.

As with all of Collyn's writings, this article is protected under the Commonwealth Copyright Act. No part may be produced in any form without the written permission of the copyright holder (Caravan and Motorhome Books, Broome WA).
Collyn Rivers, W8054