drafts:battery_life
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| ====== Five ways to extend the life of your lead acid battery. Part I ====== | ====== Five ways to extend the life of your lead acid battery. Part I ====== | ||
| - | Although high-quality batteries are expensive, they are ultra-reliable and have a long life-expectancy | + | Although high-quality batteries are more expensive |
| How long they last is directly related to how they are used …or abused. Simply knowing what you should and shouldn’t do to a battery will save you thousands – if your battery bank is large. | How long they last is directly related to how they are used …or abused. Simply knowing what you should and shouldn’t do to a battery will save you thousands – if your battery bank is large. | ||
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| In this article we’re going to look at the main causes of premature battery failure – these are: | In this article we’re going to look at the main causes of premature battery failure – these are: | ||
| - | Running a battery ‘flat …then failing to re-charge | + | - Running a battery ‘flat …then failing to re-charge |
| - | Persistent Undercharging | + | |
| - | Overcharging | + | |
| - | Charging too quickly | + | |
| - | Ignoring temperature considerations | + | |
| - | In order to understand what is going on inside a battery, we need to know how it is constructed, | + | |
| + | This article is specifically about lead batteries. There are also many other kinds of battery chemistries such as lithium, but this information is specifically about lead. | ||
| + | In order to understand what is going on inside a battery, we need to know how it is constructed, | ||
| - | There are six cells in a 12V battery – each one comprises two lead plates which are immersed in dilute Sulphuric Acid (the electrolyte) – which can be either liquid or a gel. The lead is not solid, but spongy and has to be supported by a grid. The porosity of the lead in this condition makes it fully accessible to the the electrolyte, | + | A lead acid battery cell is approximately 2V. Therefore there are six cells in a 12V battery – each one comprises two lead plates which are immersed in dilute Sulphuric Acid (the electrolyte) – which can be either liquid or a gel. The lead oxide and is not solid, but spongy and has to be supported by a grid. The porosity of the lead in this condition makes it fully accessible to the the electrolyte, |
| That chemical reaction is fairly complicated – but we need only notice a couple of things about it: As power is drawn from a battery sulphuric acid is lost from the electrolyte and combines with the lead plates to form lead sulphate. Conversely – recharging the battery forces the sulphate to leave the lead plates and return, once more, to the electrolyte forming dilute Sulphuric Acid. The second thing we need to notice is that if the charging voltage is too high, or is maintained for too long another chemical reaction begins in earnest: the water in the electrolyte decomposes into oxygen and hydrogen. | That chemical reaction is fairly complicated – but we need only notice a couple of things about it: As power is drawn from a battery sulphuric acid is lost from the electrolyte and combines with the lead plates to form lead sulphate. Conversely – recharging the battery forces the sulphate to leave the lead plates and return, once more, to the electrolyte forming dilute Sulphuric Acid. The second thing we need to notice is that if the charging voltage is too high, or is maintained for too long another chemical reaction begins in earnest: the water in the electrolyte decomposes into oxygen and hydrogen. | ||
| - | The decomposition of the water in the electrolyte into oxygen and hydrogen gas (electrolysis) is normal during the final stages of battery-charging – but is usually quite limited. Wet-cell batteries require topping-up periodically with (de-ionised) water to replace the liquid which has been lost over time. Low maintenance batteries don’t need topping-up – in fact they cannot be topped-up because they are sealed. Sealing the battery prevents the Hydrogen and Oxygen gases from escaping; instead they recombine under pressure and fall back into the electrolyte as water. Such batteries are, however, provided with a pressure-release valve in case of over-gassing – caused by charging at too high a voltage. We’ll be taking a look at charge-voltages later. | + | The decomposition of the water in the electrolyte into oxygen and hydrogen gas (electrolysis) is normal during the final stages of battery-charging – but is usually quite limited. Wet-cell batteries require topping-up periodically with (de-ionised) water to replace the liquid which has been lost over time. Low maintenance batteries don’t need topping-up – in fact they cannot be topped-up because they are sealed. Sealing the battery prevents the Hydrogen and Oxygen gases from escaping; instead they recombine under pressure, the gases are trapped |
| Driven by the movement of electrons, the cyclical passing back and forth of sulphate, between the lead plates and the electrolyte, | Driven by the movement of electrons, the cyclical passing back and forth of sulphate, between the lead plates and the electrolyte, | ||
| - | The spongy lead plates can become coated in a hard layer of sulphur | + | The spongy lead plates can become coated in a hard layer of lead sulfate crystals |
| The cohesive structure of the lead breaks down allowing some lead to fall away – this deterioration of the plates is known as ‘shedding’. | The cohesive structure of the lead breaks down allowing some lead to fall away – this deterioration of the plates is known as ‘shedding’. | ||
| | | ||
| - | These are the principle maladies which cause either an unacceptable loss of capacity in a battery, or a failure | + | |
| + | These are the principle maladies which cause either an unacceptable loss of capacity in a battery, or a failure to store or release energy at all. There are others. | ||
| To make matters worse, the functional limitations brought about by any one of these damaging events, will frequently trigger a second or third mode of failure. | To make matters worse, the functional limitations brought about by any one of these damaging events, will frequently trigger a second or third mode of failure. | ||
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| When choosing a battery size (capacity) for our job, remember that it will last longest if it is never depleted by more than half its capacity …in other words, it is never discharged below 50% state of charge (SOC). | When choosing a battery size (capacity) for our job, remember that it will last longest if it is never depleted by more than half its capacity …in other words, it is never discharged below 50% state of charge (SOC). | ||
| - | Partially discharged batteries should be re-charged as soon as it is convenient to do so. Damage is caused by leaving them in a partial state-of-charge …the lower the charge; and the longer a battery is left in a discharged condition – the greater the damage. | + | |
| + | Partially discharged batteries should be re-charged as soon as possible. Damage is caused by leaving them in a partial state-of-charge …the lower the charge; and the longer a battery is left in a discharged condition – the greater the damage. | ||
| It is safe to cycle a battery between 50% SOC and 80% SOC – it is quite efficient to do so, too. But this kind of cycling cannot be continued for extended periods. Recharging a drained battery to about 80% state of charge can be achieved quickly – but returning a battery to 100% SOC takes much longer because the rate at which it can accept charge is very much reduced as it approaches full-charge. It is important to allow the necessary charge time to return a battery to 100% SOC at least once every 30 cycles – that’s monthly for a battery which is in use every day. There are several reasons for this which we will cover a bit later. | It is safe to cycle a battery between 50% SOC and 80% SOC – it is quite efficient to do so, too. But this kind of cycling cannot be continued for extended periods. Recharging a drained battery to about 80% state of charge can be achieved quickly – but returning a battery to 100% SOC takes much longer because the rate at which it can accept charge is very much reduced as it approaches full-charge. It is important to allow the necessary charge time to return a battery to 100% SOC at least once every 30 cycles – that’s monthly for a battery which is in use every day. There are several reasons for this which we will cover a bit later. | ||
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| The cut-open battery image belongs to Sun.solanki and has been used without alteration. | The cut-open battery image belongs to Sun.solanki and has been used without alteration. | ||
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| So much for discharging too deeply: If the battery is then left in a discharged condition the tiny crystals of sulphate which have formed begin to grow. The sulphate on the surface of the plates begins to harden – eventually forming themselves into an impenetrably hard white coating around the lead plate, which plugs the porosity of the material – and greatly impedes the diffusion of ions which drive the chemical process. By this stage the battery’s capacity, and its ability to accept or release energy will be so slow that it will be unable to do the work for which it has been chosen. | So much for discharging too deeply: If the battery is then left in a discharged condition the tiny crystals of sulphate which have formed begin to grow. The sulphate on the surface of the plates begins to harden – eventually forming themselves into an impenetrably hard white coating around the lead plate, which plugs the porosity of the material – and greatly impedes the diffusion of ions which drive the chemical process. By this stage the battery’s capacity, and its ability to accept or release energy will be so slow that it will be unable to do the work for which it has been chosen. | ||
| - | This kind of battery damage occurs when, for example, a vehicle’s headlights have been left on, and the vehicle remains unused for a period of days or weeks …or a battery has been left on a shelf in a workshop for a period of months, and it has self-discharged until it is flat. Almost undoubtedly, | + | This kind of battery damage occurs when, for example, a vehicle’s headlights have been left on, and the vehicle remains unused for a period of days or weeks …or a battery has been left on a shelf in a workshop for a period of months, and it has self-discharged until it is flat. Almost undoubtedly, |
| - | If any of the damage is reversible, it can be reversed by recharging the battery in the normal way (if it will recharge), and then applying an equalisation charge until the battery voltage reaches 16V or 17V (for a 12V battery) for a period of, say, three hours. This will force the sulphated areas of the plate to release the sulphate back into the electrolyte. Success is not guaranteed. | + | If any of the damage is reversible, it can be reversed by recharging the battery in the normal way (it may be slow if it will recharge), and then applying an equalisation charge until the battery voltage reaches 16V or 17V (for a 12V battery) for a period of, say, three hours. This will force the sulphated areas of the plate to release the sulphate back into the electrolyte. Success is not guaranteed, and in nearly all cases there will be some permanent capacity loss. |
| + | |||
| + | Be very careful to monitor the battery closely at these high charge voltages, as this will also be causing the electrolyte to separate into gas. | ||
| ===== Charging a battery too quickly ===== | ===== Charging a battery too quickly ===== | ||
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| A battery should be charged with a current no greater than 20% of it’s capacity. For example, if the battery has a 100 amp/hour rating, its maximum charge current should be no greater than 20amps. A discharged battery is able to accept much higher rates of charge – for a short time – but this kind of charging should be avoided. High-output alternators, | A battery should be charged with a current no greater than 20% of it’s capacity. For example, if the battery has a 100 amp/hour rating, its maximum charge current should be no greater than 20amps. A discharged battery is able to accept much higher rates of charge – for a short time – but this kind of charging should be avoided. High-output alternators, | ||
| - | At first this shedding ‘only’ reduces the battery’s capacity – later, as lost material accumulates at the bottom of the battery, it will eventually touch both the positive and the negative plate, the cell will not function, and the battery will lose the voltage from that cell (failure of the other cells will not be far behind). | + | At first this shedding ‘only’ reduces the battery’s capacity – later, as lost material accumulates at the bottom of the battery, it will eventually touch both the positive and the negative plate creating a short and the cell will not function. The battery will lose the voltage from that cell (failure of the other cells will not be far behind). |
| An exacerbating factor, with charging a battery too quickly, is that fast charging increases the battery’s temperature. The controlled charge cycle for a particular battery – the voltages at which it is charged during each of its three charge-phases – have been calculated with the assumption that the battery temperature is 20ºC (usually) | An exacerbating factor, with charging a battery too quickly, is that fast charging increases the battery’s temperature. The controlled charge cycle for a particular battery – the voltages at which it is charged during each of its three charge-phases – have been calculated with the assumption that the battery temperature is 20ºC (usually) | ||
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| ===== Repeated Failure to fully re-charge a battery ===== | ===== Repeated Failure to fully re-charge a battery ===== | ||
| - | |||
| Most of us monitor the state-of-charge of a battery by the rough and ready method of ‘observing battery voltage’. In the fast-charge installation imagined above, for example, voltages climb so quickly that it gives us the illusion that our battery is fully charged, and that we can therefore terminate the charge cycle believing that the job to be nearly done. Although batteries charged and discharged in this way are actually more ‘efficient’ (in that most of the energy offered to the battery is absorbed by the battery) – short sharp charge cycles result in persistent undercharging. Repeated undercharging causes three problems: | Most of us monitor the state-of-charge of a battery by the rough and ready method of ‘observing battery voltage’. In the fast-charge installation imagined above, for example, voltages climb so quickly that it gives us the illusion that our battery is fully charged, and that we can therefore terminate the charge cycle believing that the job to be nearly done. Although batteries charged and discharged in this way are actually more ‘efficient’ (in that most of the energy offered to the battery is absorbed by the battery) – short sharp charge cycles result in persistent undercharging. Repeated undercharging causes three problems: | ||
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| - | when selecting a battery, make sure it is the correct type for the work it has to do …engine starting, or deep cycle; standby-power, | + | When selecting a battery, make sure it is the correct type for the work it has to do …engine starting, or deep cycle; standby-power, |
| Make sure the battery bank has the capacity to serve its purpose easily. In practice, for long life, this means specifying a capacity around four times the requirement. | Make sure the battery bank has the capacity to serve its purpose easily. In practice, for long life, this means specifying a capacity around four times the requirement. | ||
| Ensure that the battery duty cycles includes a period when the batteries can be brought slowly up to 100% state of charge and allowed time beyond that so the the cells can equalize. This should be at least once every 30 cycles. | Ensure that the battery duty cycles includes a period when the batteries can be brought slowly up to 100% state of charge and allowed time beyond that so the the cells can equalize. This should be at least once every 30 cycles. | ||
| Installing an automatic load shut-down device in order to prevent draining a battery below, say 20% SOC, may be the best investment you can make. | Installing an automatic load shut-down device in order to prevent draining a battery below, say 20% SOC, may be the best investment you can make. | ||
| - | Sulphation: Lead and lead-dioxide react with sulphuric acid to form lead sulphate – small crystals which easily reforms back to lead, lead-dioxide and sulphuric acid. After time, some lead sulphate does not revert, but forms a stable crystalline coating which no longer dissolves on recharging. Sulphation can be avoided | + | Sulphation: Lead and lead-dioxide react with sulphuric acid to form lead sulphate – small crystals which easily reforms back to lead, lead-dioxide and sulphuric acid. After time, some lead sulphate does not revert, but forms a stable crystalline coating which no longer dissolves on recharging. Sulphation can be reduced |
| The overblown battery image belongs to Dennis van Zuijlekom | The overblown battery image belongs to Dennis van Zuijlekom | ||
| Justin Tyers | Justin Tyers | ||
| + | |||
drafts/battery_life.txt · Last modified: 2019-01-22 10:42 by 127.0.0.1