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Battery Charging primer
WotEver posted a topic in Boat Building & MaintenanceI wrote this elsewhere but as the same questions pop up regularly I thought it might be an idea to post it here. I don’t intend this to be an exhaustive in-depth ‘instruction manual’ which covers all aspects of marine battery charging, there are already many web sites and books which attempt to do that. Rather, this describes, in pretty broad brush strokes, my opinion of how to approach the matter while taking into account your boating style. It’s also not written with substitute examples (like buckets for batteries) but with real products that you may have on your boat.While I’ve tried to avoid being overly technical whilst describing what is a highly technical subject, there are nevertheless mentions of volts and AmpHours here. Despite that, I’ve intended it to be reasonably light reading for those with a little knowledge of the subject yet possibly containing some nuggets of useful information for those who know the subject quite well. It's also worth pointing out that there is no such thing as 100% charged; by the very nature of its chemistry a battery can never get there. You can always squeeze just a tiny bit more in. However I will refer to 'fully charged' as meaning "as charged as practical" !Please read the couple of safety points at the end of this post. It’s more than possible that I’ve overlooked something or made errors in which case I’m sure you’ll let me know . So, here we go.Battery charging is a topic that generates almost more discussion on any boating forum than anything else. This article tries to set out the basics for keeping your batteries healthy and happy, but can only be an introduction to such a complex business. Let us first understand the problem. When sitting in the cabin with no engine or generator running, you are consuming power from your batteries. TV, radio, lighting, pumps, phone chargers etc. all use power. Now, if you cruise for a number of hours the following day then it is possible, depending on how many hours that your engine is running, that your alternator will have replenished the lost charge by the time you moor up for the evening. Then you start the cycle again.In the summer a good solar installation can take care of some or even all of your charging needs (depending on the sunshine and the size of the solar panels). In the winter solar will not achieve this. In fact you will experience days or even weeks where the solar contributes virtually nothing at all. Wind generation is of even less value.If you require extra battery charging then the solution is therefore a mains battery charger. What type, how it is used, and how it should be configured is covered in the following sections.Before we get to that, let us first understand what a modern, microprocessor controlled battery charger does. All modern battery chargers have three basic stages. Marketing departments will sometimes advertise 4, 5, or even 6 stages of charging but that is purely so they can make their charger appear superior to the competition. For a straightforward recharge of discharged batteries there are only three stages. (Equalisation or Desulphation are not a charge stage, they are a maintenance stage). So, what are these three stages?1. Bulk Stage. This is when the batteries are at their lowest SoC (State of Charge) and will 'demand' the maximum output from the charger. All chargers are categorised by their maximum current rating (20A, 40A etc) and during the Bulk Stage the current will be at this maximum while the voltage slowly rises up to the preset maximum (typically 14.4 to 14.7V, depending on battery type and internal chemistry). This stage is sometimes called "Constant Current". Note that the size of the charger (its maximum current) should be chosen to suit your usage, the size of the battery bank and the battery type. How this is arrived at is outside the scope of this post.2. Absorption Stage. The batteries have now reached a higher SoC (typically around 80-85%) and are gradually demanding less and less from the charger. The voltage remains at the preset level (typically somewhere between 14.4v to 14.8V) whilst the current slowly reduces. This stage is sometimes called "Constant Voltage". This stage should continue until the batteries are very close to 100% charged.3. Float Stage. Depending on your charger's float voltage this can be considered to be similar to the Absorption Stage inasmuch as the voltage is constant, but the voltage is now reduced to around 13.6V (again, preset according to battery type and internal chemistry) in order to treat the battery gently and to slowly bring it to a fully charged state whilst simultaneously countering self-discharge. Some chargers, however, have a much lower float voltage of only around 13.25V. If yours is one of these then float should be considered as a maintenance stage because it will not charge the battery, only keep it at its current state of charge.Self-discharge is where a charged battery will slowly lose its charge if left unattended.How a battery charger should be used and configured depends largely upon your boating style, which is discussed below.There are three main scenarios for using the batteries which will be taken in turn.If you are off-grid with no access to shoreline If you never have an opportunity to plug your boat into a shoreline and solar/wind is insufficient then you are reliant on either the engine alternator or a TravelPower or separate generator to feed an on-board charger. Whichever of these two methods you use (engine or generator), you will be limited in how long you can run it. CRT licence conditions do not permit the running of engines or generators for charging purposes outside the hours of 8 am to 8 pm and in any event, it is highly inconsiderate to spoil the peace of an evening on the cut with engine noise. Besides, why add wear to the engine and use fuel any more than is essential?Note that thanks to something called Charge Efficiency (don't worry about it, just accept this), more energy needs to be replaced when charging than you took out when discharging (by a factor of around 10-20%). It is impossible to tell from a simple volt meter when the bank is fully charged. In brief, you will need to monitor either the specific gravity (relative density) or an ammeter to know when it is safe to stop charging. This is particularly pertinent to off-grid boaters who for reasons of noise and expense of fuel do not wish to charge for hours on end when unnecessary.If using the SmartGauge to monitor your batteries, be aware that it is not as accurate when charging as discharging and therefore continuing charging for some time after it shows 100% may be advisable. If using a monitor that contains an ammeter, you need to watch the current flow into the battery bank. It will start very high during the bulk phase, then tail off during absorption. When it has either stopped reducing over a period of an hour or reached 1 - 2% of the bank's capacity, then little will be achieved by continuing charging. So for a 400 Ah bank, for example, you should aim for a "tail current" of 4 - 8 amps if it is achievable.There is however a problem with most (if not all) battery chargers. When charging with a generator and 'mains' battery charger it is very important to note that many chargers switch to float voltage much too soon. This is because the designer was imagining them being used on a never-ending household mains supply, and switching to float early treats the batteries gently at the expense of charging time. An off-grid boater needs the opposite; he needs the batteries to charge as fast as possible so that he can switch off his generator as early as possible. Ideally, an off-grid boater doesn't want the charger to ever switch to float; simply continue at absorption voltage until the tail current indicates that the bank is fully charged. Note however that this requires the boater to be vigilant. Continuous charging at the Absorption voltage once the batteries are charged will irreparably damage your batteries.Some chargers permit the user to configure the settings to a large degree, and I would advocate, where possible, that an off-grid boater should set the float voltage to be the same as the absorption voltage - somewhere in the order of 14.4V depending on the make of battery. It is important to repeat the advice that if you do this you must be vigilant and stop charging once the batteries are close to 100% SoC.Although somewhat inconvenient and requiring experience, it is possible, once charging has finished, to use an accurate voltmeter to estimate the state of charge, but only once the surface charge has been eliminated. One way of doing this would be to turn the tunnel light on for around 10 minutes. This table shows the relationship between resting voltage and charge state of a typical 12V bank:12.65V - 100%12.45V - 75%12.24V - 50%12.06V - 25%11.89V - 0%The highly accurate way to check the state of charge of individual cells is to use a hydrometer or refractometer to measure the relative density of the electrolyte but many boaters will not wish to go to those lengths and those with sealed, AGM or gell batteries will not be able to do so anyway.You should aim to fully charge the batteries daily for maximum life. If that is impossible, then every few days, certainly every week, to avoid sulphation permanently reducing the bank's capacity.If you are off-grid with occasional access to shoreline If you are able to get into a marina occasionally and plug into a shoreline, then you can give the batteries a much more thorough charging. Leaving the charger on until it goes into float mode, turning it off for a while then back on, will result in the batteries being brought to a much better state. Ideally, leave the charger on overnight.If you have configured the Float Voltage to be high as described above for off-grid boaters then it is essential that you change it back to a suitable float voltage for your batteries when connected to shore power.If you have regular access to shoreline If you regularly moor in a marina or somewhere else where you can use a shoreline, then in addition to all of the foregoing you can routinely leave the bank on float charge. Most modern chargers will go back into absorption mode from time to time to ensure that the batteries are kept in good condition.During the summer, a decent solar panel array will achieve the same effect, keeping a float level of charge across the bank, as long as your power usage is less than that which the bank is producing, but the weak UK winter sun will not be adequate for this purpose.EqualisationYes, I know it's not a 'charge mode' but I thought I'd add a little about it here anyway. Some chargers will enable you to give an equalisation charge at a much higher voltage than usual, probably above 15V for a 12 volt system. This has the effect of ensuring that all the cells in the bank have been fully charged and that as much sulphation as possible has been removed from the plates. Some warnings about equalisation: 1. Sealed, AGM and gel batteries should not normally be subject to equalisation. If you are considering doing so, take expert advice first.2. Some items of equipment in the boat may not be able to handle the high voltage and should be disconnected. The safest way is to isolate the bank before proceeding.3. Hydrogen gas will be given off during the process. The bank should be well ventilated and must never be left unattended. During equalisation, check the temperature of the batteries from time to time, say every 15 minutes. They may well become warm to the touch but serious heating is a bad sign and the process should be stopped at once. It is not unknown for batteries to boil or even explode if a cell has failed.4. The cells will need topping up with distilled water after being equalised.A couple of general bits of safety advice should be added here:1) A permanently installed charger must never use crocodile clips to connect to the battery. 2) Temporary chargers should only be connected with the input (230V mains) power turned off, and only disconnected 3 minutes after turning off the charger input power to minimise the risk of explosion from hydrogen gas generated during charging.
AWETHEAYET posted a topic in Boat Building & MaintenanceDon't know if this is the correct place to ask my question, there may already be a post that covers it. but here goes. I want to convert a future boat to hybrid, to make it all electric onboard and propultion by both engine and battery. What size invertor do I need for this task? what battery monitoring is required? What battery bank will I need? what generator / motor is available? (No limit cost and budget alternatives for the system). The origion post is one of the best I have read on battery maintenance and functionality and thank you for the education.