Batteries - Revised
Vega
Hugh and Annie
Tue 22 Mar 2022 09:02
| This is a subject about which I have written very little. The only thing I really know about batteries is that they are essential, expensive and need to be kept fully charged. I know even less about electrics generally but can recommend the RYA Electrics Handbook to anyone wishing to learn more. We have one battery dedicated to starting the engine and three “house” batteries to power everything else. The starter motor battery is what you might expect to see in a car and is good at providing short bursts of high power but useless at sustaining even modest output for any length of time. This is why it will start your engine in the morning but run down overnight if even a small light is left on. Our three house batteries are (or should be) designed to maintain a relatively low output over a longer period and are what are known as “deep cycle” batteries that can be run down and recharged many times. This is useful on a boat where the recharging may be erratic e.g. non existent at night or at a high level when the engine is running. The capacity of a battery is shown as Amp Hours (Ah) which means the higher the Ah the longer it will provide an output until it is flat. Our engine battery is 75Ah whereas the house batteries are each 120Ah. 120Ah means the number of amps you might expect from the battery if it is discharged over a standard time (usually 20 hours). In practice the draw on a battery is likely to result in discharge over a shorter or longer period. The relationship between the amount of draw and the time remaining before the battery is flat is not linear. The higher the draw the faster the rate of reduction of capacity (and vice verca). The engine and house batteries are charged on the same circuit but are on separate circuits for power supply. They can, however, be switched into the same circuit in an emergency so that if, for example, the starter battery fails the house batteries can be used to start the engine. Now, there are lots of complications concerning battery management and charging. We replaced all the batteries in Thailand and the total capacity of all the batteries is now 435Ah. The house batteries total 360Ah. Given that the rated output of the engine alternator should be, rule of thumb, 25% of the total battery capacity our alternator at 80A is either slightly or significantly below this, depending upon whether you include the starter battery in the calculation. As discussed in a previous post, too small an alternator can cause excessive wear on the alternator belt if the batteries are low and demanding more amps for recharging than the alternator can supply. This should rarely be a problem if we keep the batteries well charged. There are however some basic rules that must be followed and to do this battery performance is monitored. Ideally the capacity of a battery should not be allowed to drop below 50% and it should not, if at all possible, be allowed to go completely flat (less than 12V) as this can cause permanent damage to the lead plates. The voltage of the batteries is monitored; 12.2V is considered to represent 50% capacity and this is the critical figure for many yachties. 12.4V is 75% and an ideal minimum to aim for. A battery monitor may also calculate the Amp Hours used and the remaining capacity. It will recalculate from whenever the battery is full and express the capacity as a percentage. Full is 100%. There is a formula for calculating remaining capacity that includes a Peukert number (named after its inventor). This Peukert number varies depending upon the efficiency of the battery being monitored. A good quality battery may have a Peukert number as low as 1.1; a poor quality battery may have a number as high as 1.8. Whatever it is, this number can be entered into the battery monitor to be used when calculating the remaining capacity of the batteries concerned. The Peukert number entered into our Mastervolt battery monitor is 1.23. When batteries are being charged they will show a voltage of over 13V and this should not exceed 14V unless it is a battery where a high voltage can be good for mixing the electrolyte from time to time. A regulator will ensure this but without a regulator to control the charge the voltage will need to be monitored and charging stopped if the voltage gets to +14V. On a boat there are different options for recharging the batteries. When plugged into shore power we have mains electricity feeding a battery charger that has a sophisticated charge profile depending upon the state of battery charge. When the batteries are full the charger will maintain a light “float” charge to keep them full but without over charging. Just like a car, every time we use the engine the alternator will charge the batteries. When the alternator comes on the charge rate may go quickly up to 40 Amps or thereabouts depending upon the level of battery charge. This charge rate will reduce as the batteries become fuller and will come down to 2 or three amps when the batteries are full. For coastal cruising and short passages between marinas, shore and engine power are likely to be all that is required. For offshore cruising there may not be shore power for months at a time. Furthermore there is likely to be more electrical equipment and much more use of electrical equipment than on a standard coastal boat. Given that rechargeable batteries do not generate electricity, they simply store the current that is put into them, a process of continuous charging may be required at times. Without shore power and even when under sail the engine can be used to charge the batteries. However, in order to conserve fuel, save engine hours and generally avoid using the engine, alternative forms of power generation are used. Solar panels are now very efficient and affordable. We have two, each rated at up to 5 Amps output. We recently had them tested and they are each still producing 4.5 Amps in the brightest midday sunlight. In practice they will rarely achieve this due to orientation with the sun, time of day, cloud cover and so on but nevertheless they are still working as they should. They have their own dedicated regulator for the current going to the batteries. In addition we have wind and hydro powered generation. In our case this is in the form of our Duogen that converts from wind power when we are at anchor to hydropower when sailing. Separate wind and hydro generators are more common but nevertheless the Duogen is popular and is particularly good in hydro mode when it will produce up to 11 Amps at 7 knots boatspeed. In wind mode it is less efficient than dedicated wind generators but should provide two or three amps of charge in a good breeze. So, we have five potential sources of power generation. Our regular generation is dependent upon the wind and sun. We have generation when the engine is running and from a turbine when sailing. Finally we have generation from shore power when in a marina. The amount of recharging required is determined by the amount of draw on the batteries from the electrical items on the boat. The biggest ongoing draw is from the ‘fridge which is the only piece of equipment on all the time. This draws about 2.9 Amps when running but switches on and off as required to maintain the preset temperature. Sometimes we will raise the temperature setting to reduce power consumption if the batteries are getting low at night. When we are sailing with all the navigational instruments, autopilot, VHF radio, nav lights and so on running we may be consuming seven or eight amps and this is why our Hydrovane self steering is important - it means we don’t need to use the autopilot (or hand steer). The radar is a big draw on the batteries, another six or seven amps, and so we only use it sparingly when sailing but it is also why having the Duogen running in turbine mode on passage is so important - without the Duogen we struggle to get through a night without having to run the engine. This has all now become more relevant for three reasons: 1. The regulator for the Duogen is dumping output into the resistors without feeding it into the batteries. This should only be the safety mechanism to prevent over charging and means that in either mode we are only getting between 1 and 2 amps into the batteries. I have a by-pass switch to install (when we catch with Jon on Hecla who is bringing it from Uligan) that will allow us to feed directly into the batteries. 2. The regulator for the solar panels appears to be suppressing input from the Duogen in wind mode when the solar panels are active. There is a reason for this that I have long since forgotten but I also have concerns about the the solar panel regulator. 3. Now that we are tied up with no engine and no shore power we are reliant upon the wind and solar generators. With very little wind at this time of year we have no charging at night and are therefore totally reliant upon the solar panels during the day. A cloudy day could mean trouble but so far the sun has remained amazingly intense. We have been carefully monitoring the status of the batteries and keeping a record throughout the day. This has been remarkably consistent and can be summarised as follows (with the range shown for different days): 0700 Capacity 98% 12.5V +0.2A Capacity 96% 12.4V -2.3A 1200 Capacity 98% 13.0V +3.0A 1530 Capacity 99% 13.1V +0.9A Capacity 100%. 13.1V +0.7A 1800 Capacity 100% 13.0V +0.3A Capacity 98% 12.6V -4.1A For the last four days the capacity has shown 100% by late afternoon. Recently at 1500 the capacity was 100% and the voltage 14.28V! This is unusually high and is one reason to suspect the solar regulator. We turned the ‘fridge temperature lower to use more power and reduce the battery voltage! According to our records the capacity since we have been here has not dropped below 96%. And yet the voltage readings have suggested that the capacity has actually been as low as 75%. I do not understand this inconsistency and can only think the Peukert number entered into the Mastervolt is incorrect for our new batteries. On the regulator for the solar panels there should be two green or one green and one red LED lights showing. If the unit is operating correctly a green light will illuminate (and it is). In addition a second LED should indicate the state of charge of the batteries - float ie full, high, low or flat (red). A second light is not illuminating. The Mastervolt monitors the battery voltage from a “bus” or resistance in the battery circuit which is part of the Mastervolt setup. I am assuming this is giving a true reading and from this we may be being too frugal in the use of our electrical equipment (shower pump for example that uses a lot of amps when running) because the batteries are never below 75% (based upon the voltage readings). On the other hand I am surprised the batteries are charging so quickly from the solar panels. And why is the voltage reading fluctuating so much more than the capacity percentage? If there is anyone out there who knows more about these things than I (probably most of you) then I would be very grateful for feedback. However, since writing and first posting this piece I have done some more investigation of our new batteries and think I know why the battery monitoring is not what might be expected. The new batteries we were supplied in Thailand are not deep cycle batteries - they are in fact high capacity car batteries (designed for the likes of Rolls Royce and other large power hungry cars but not for boats). This is why the voltage fluctuates so much with use and why they respond to continuous charging in the way they do. I am trying to get a response from the marine electronics company that supplied them on how to best manage them. We either persevere with them and use them in a way not intended which probably means a shorter life than normal or we replace them with deep cycle batteries that it looks like we can source in Male.   |