Caring for Deep Cycle Batteries

Warning - Fire Hazard

warning sign: corrosive acid

A low voltage power supply is just as likely to cause a fire if a short circuit occurs as with any other voltage. Please use suitable fuses or circuit breakers near the battery and between the battery and any other power sources. Also ensure that electrical conductors such as metal objects cannot accidentally fall across the battery terminals.


Batteries generate explosive gases during operation and when charging. Flames, sparks, burning cigarettes or other ignition sources must be kept away at all times. Ensure that there are no loose metal objects around the batteries that can be blown down by a strong wind or knocked onto the battery terminals. Similarly sparks can be generated at the battery due to a poor connection. Always shield eyes when working near batteries. Battery charging should be carried out in a well ventilated area - never in a closed room. Always turn battery charger off before disconnecting a battery.

Warning - Acid

sketch: wash your hands

Battery acid (or electrolyte) can cause burns. Use extreme care when handling acid. If electrolyte is spilled or splashed onto clothing or the body, wash with water and neutralise with a solution of baking soda and water. Electrolyte splashed into the eyes is extremely dangerous. If this occurs, force eyes open and wash with clean cool water for five minutes and call a doctor. A solution of 1 tablespoon of bicarbonate of soda to ½ litre of water should be kept readily available and in view near the battery bank. This solution will neutralise the acid and hence be a more effective eye-wash in the event of such an accident.

Topping up your Battery

The water used for diluting acid and for topping up must be free of mineral impurities. Distilled water or demineralised water may be used. Never use tap water as the effect of impurities is cumulative and detrimental to the battery.

NOTE: Do NOT add battery acid to the battery unless under the specific directions of a battery technician.

Moving Batteries

Never lift your batteries by their terminal posts.

Placement of Batteries

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Place batteries on a firm, solid and level support. Weight of batteries should be equally distributed over the base area. Batteries should not be in direct contact with a cold surface such as concrete. If the base of the battery stays cold, the acid will not mix readily and will tend to stratify (most concentrated acid at the bottom and least concentrated at the top). It is recommended that you use an insulating material such as rubber or vinyl under the batteries that will not be affected by the corrosive properties of battery acid.

Battery Connections

Make as few connections directly to the battery as possible. It is desirable to have a fully fused Distribution/Meter Box from which all other connections can be made.Before making your connection to the battery, first smear petroleum jelly (e.g. Vaseline) over the terminal post to prevent or reduce the likelihood of battery acid creeping up the terminal post and rapidly corroding your connector.

Make sure that the connector is fixed very firmly and that it is making good contact with the terminal post to reduce voltage drop. Do not increase your battery capacity by connecting several small batteries in parallel. The more parallel connections there are, the more prone the system is to uneven charging due to lazy cells and unequal cell characteristics. This will cause an overall reduction in expected battery life and increase maintenance requirements.

Charging and Discharging of Batteries

The life of a battery is related to how many times and how deeply it can be cycled (charged and discharged). A cycle is defined as one charge, to fully charged, and one discharge, to almost fully discharged. An 80% discharge is regarded as 'deep'. However, for maximum life, lead-acid batteries should be discharged as little as possible. We recommend that your average cycle should be no deeper than about 20%, and never beyond 50%.

A standard car battery can only take about twenty deep discharges before it becomes completely useless.

If you have an all year round hydro potential then you may get away with only a very small battery bank (equal to your daily usage) because the battery bank is getting charged 24 hours per day. If you only have an intermittent flow, then a hydro system may be an excellent back-up for a solar power system.

Visual Inspection

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Check electrolyte level at least once a month. If the batteries are fully charged and still charging, water loss may increase. It is advisable that a suitable charging regulator be installed to prevent overcharging of the battery. Overcharging is indicated if the battery is bubbling vigorously.

Hydrometer Test

Check the electrolyte level, to ensure that it is above the plates in all cells.

If it is below the plates, the test cannot be carried out until water is added and the battery charged to mix the water and residual acid in the battery. It is important to ensure that the plates do not remain exposed to air and allowed to dry and oxidise.

The state of charge of each cell can be measured with a hydrometer to determine the specific gravity of the electrolyte (specific gravity is its weight compared to water).

Using a Hydrometer

sketch: using a hydrometer to check battery

Draw the acid into the hydrometer, so that the float is lifted free and not touching the top or the bottom. The barrel must be held vertically and the eye level with the surface of the liquid. Disregard the curvature of the liquid against the glass.

Cell temperature corrections should be applied if accurate readings are required. 0.004 points should be added or subtracted for each 5 C +/- variation from 25° C.

Voltage Test

Voltage readings should be taken whilst the batteries are neither charging nor discharging (nothing connected and turned on). Immediately after either charging or discharging the battery voltage may not have stabilised. The voltage will settle down in about 30 minutes after charge or discharge are discontinued.

Rainbow Power Company can supply you with battery connectors, distribution box, fuses, suitable electric cable, charging systems etc. Do not hesitate to contact us for more advice, information, service etc.

SOCState of Charge
100% 1.277 2.12 1.240 2.086 1.225 2.0950
90% 1.258 2.10 1.230 2.077 1.216 2.0775
80% 1.238 2.08 1.220 2.067 1.207 2.0600
75% 1.227 2.07 1.215 2.062 1.203 2.0513
70% 1.217 2.06 1.210 2.058 1.198 2.0425
60% 1.195 2.04 1.200 2.048 1.189 2.0250
50% 1.172 2.02 1.190 2.040 1.179 2.0075
40% 1.148 2.00 1.180 2.031 1.171 1.9900
30% 1.124 1.98 1.170 2.022 1.163 1.9725
25% 1.111 1.96 1.165 2.018 1.158 1.9638
20% 1.098 1.95 1.160 2.013 1.153 1.9550
10% 1.073 1.93 1.150 2.005 1.145 1.9375
0% 1.048 1.91 1.140 1.996 1.135 1.9200

SG* = Specific Gravity at 25° C
OCV† = Open Circuit Voltage per 2 Volt Cell

We install solar systems in Northern NSW and Southern QLD.

Gold Coast (from Coolangatta to Southport), Nerang and Hinterland (Beaudesert) and out West (Warwick, Stanthorpe, Killarney)

Northern NSW (Tweed Heads to Yamba, including Evans Head, Byron Bay and Ballina); the Far North Coast Hinterland (Grafton via Lismore to Murwillumbah) and out West (Casino to Tenterfield, including Drake and Tabulam, as well as Woodenbong and Bonalbo)

For larger system we also go up to Brisbane or down to Coffs Harbour and even Glen Innes. Other places by arrangement.

Tesla Powerwall now in stock!

Rainbow Power Company is pleased to announce that Tesla Powerwall Home Batteries are now in stock - ready to be installed.

Tesla Powerwall is the most cost effective and innovative grid-storage battery on the market and can be retrofit to most existing solar systems.

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It may sound like a dull sales slogan but stock is limited and moving out fast.

Contact us today for an obligation free quote.