31-07-2022, 09:37 PM
There is a confusing array of choices when you want a battery. This article considers lead-acid batteries used as habitation batteries in motorhomes.
A battery consists of a few components: Firstly, the casing, which is the box that holds everything. It will have some sort of filling and vent port (you don't want pressure to build up and cause an explosion}. Second, the plates, which are the panels that convert electrical energy to and from chemical energy. Thirdly, the electrolyte, which is the "battery acid" that fills the battery. Fourthly, the frame and connections. The plates have to be held firmly in place and connected electrically to each other and to the battery terminals. The frame has to be strong and resistant to corrosion, vibration and shock.
Lead-acid batteries have been around for a long time, and are available in many variants. All of them have the same basic essentials: lead plates in a container full of an electrolyte which is basically dilute sulphuric acid.
The plates are made of slightly spongy lead so that the elecrolyte can soak into the plates, allowing the charge to be stored inside. Thicker plates hold more charge, but are less able to deliver high currents, becasue the surface area to volume ratio is lower.
In recent years, lead carbon plates have become popular. These have carbon mixed into the negative plates, which makes them work better (with a lower charging volrage, which makes them more efficient), and reduces the buildup of lead sulphate. I think they also reduce corrosion of the positive plate, though I'm not personally clear about how that is. Can anyone explain?
Battery plates are heavy, and need to be held rigidly, not allowed to touch one another. They need to be well connected to the terminals to take the power in and out.
Each cell holds a voltage of 2v, so for a 12v battery, there are six cells inside. There may be several plates wired in parallel in each cell, but the six cells are wired in series between the battery terminals.
When a battery passes fully charged, it starts to gas. Basically, it is the electrolyte being turned into (explosive) gas by the excess electricity.
Decades ago, batteries had to be topped up with water (and sometimes with acid) to make up for this loss. Nowadays, batteries tend to be sealed.
Some simply have a large space at the top above the plates (these are known as Sealed Lead Acid (SLA) and can be prised open and topped up when needed, but generally sealed batteries are designed to contain a bit of pressure, so these gases don't escape, but simply turn back to electrolyte over time. These batteries are called Valve Regulated Lead Acid (VRLA) and have a safety valve designed to release dangerous pressure. You can't and shouldn't open them to top up.
There are three forms of electrolyte: liquid dilute acid (which has been used for decades, and is known as "wet cell"), gel (which is basically liquid with a thickener to make it unspillable) and Advanced Glass Mat (AGM). This is more or less a paste version of the electrolyte, embedded in a glass-fibre mat. AGM and (particularly) gel usually need to be charged at lower speeds to prevent build-up of gas around the plates.
Normally, gel or AGM batteries are in VRLA cases, but not necessarily all of them are.
The advantage of gel is that you can turn the battery upside down with no risk of spillage. Ths might be an advantage for a caravan (where people carry batteries out to charge them), but I can't see it being useful for a motorhome. Because of selling in smaller numbers, gel batteries cost more. Some people seem to think that means they are better. That's not my opinion!
AGM batteries do cost more, and they do have advantages. When a battery is used in a stationary setting (such as a parked motorhome) it is possible for stratification to occur, so that different levels of the battery are at different states of charge. Apparently, having the electrolyte as a paste between the plates prevents this. The other advantage is that the glass fibre mat between the cells gives additional physical support to the plates. AGM batteries do cost more than wet cell. I'm not sure if the extra cost is worthwhile, but AGM VRLA batteries are the ones I have had best results from, and are what I would choose.
So once you have decided which sort of battery to buy, which one should you choose?
Firstly, remember that these are heavy things full of acid. They need to be very well secured inside the vehicle. In the event of an accident, you really don't want a plastic box fulled with acid, lead, explosive gases and a source of sparks flying around in the van. So you are limited to what will fit properly into a strong retaing clamp: probably the one the motorhome builder fitteded.
I've seen batteries held in place by 9mm of MDF board. I shudder to think of what might happen in an accident.
It is well worth fitting the battery with the biggest capacity there is space for. The bigger the battery, the less depleted your load will make it. And if you have two batteries, wire them in parallel to share the load, rather than keeping one as a spare.
Once you have decided the type and the size, how do you choose which make and model to buy? That's the easy bit! Buy the heaviest. The amount of lead is the important factor. Lead is by far the heaviest thing in the battery (unless it is filled with gold: if so, buy it anyway!)
Assuming a 12v 100Ah battery, a good one will weigh 30KG or more. An acceptable one will weigh about 25Kg. If it weighs 20KG or less, choose something else.
One caveat: we have two 12v 240Ah deep cycle batteries. They weigh in at 75KG each. This works out at over 31 KG per 100Ah, which is good, but frankly, 75KG is very heavy and difficult to deal with. Two batteries of half the weight would have been far easier (but these were an unmissable bargain). Heavier is good for a battery, but not for payload or handling.
A battery consists of a few components: Firstly, the casing, which is the box that holds everything. It will have some sort of filling and vent port (you don't want pressure to build up and cause an explosion}. Second, the plates, which are the panels that convert electrical energy to and from chemical energy. Thirdly, the electrolyte, which is the "battery acid" that fills the battery. Fourthly, the frame and connections. The plates have to be held firmly in place and connected electrically to each other and to the battery terminals. The frame has to be strong and resistant to corrosion, vibration and shock.
Lead-acid batteries have been around for a long time, and are available in many variants. All of them have the same basic essentials: lead plates in a container full of an electrolyte which is basically dilute sulphuric acid.
The plates are made of slightly spongy lead so that the elecrolyte can soak into the plates, allowing the charge to be stored inside. Thicker plates hold more charge, but are less able to deliver high currents, becasue the surface area to volume ratio is lower.
In recent years, lead carbon plates have become popular. These have carbon mixed into the negative plates, which makes them work better (with a lower charging volrage, which makes them more efficient), and reduces the buildup of lead sulphate. I think they also reduce corrosion of the positive plate, though I'm not personally clear about how that is. Can anyone explain?
Battery plates are heavy, and need to be held rigidly, not allowed to touch one another. They need to be well connected to the terminals to take the power in and out.
Each cell holds a voltage of 2v, so for a 12v battery, there are six cells inside. There may be several plates wired in parallel in each cell, but the six cells are wired in series between the battery terminals.
When a battery passes fully charged, it starts to gas. Basically, it is the electrolyte being turned into (explosive) gas by the excess electricity.
Decades ago, batteries had to be topped up with water (and sometimes with acid) to make up for this loss. Nowadays, batteries tend to be sealed.
Some simply have a large space at the top above the plates (these are known as Sealed Lead Acid (SLA) and can be prised open and topped up when needed, but generally sealed batteries are designed to contain a bit of pressure, so these gases don't escape, but simply turn back to electrolyte over time. These batteries are called Valve Regulated Lead Acid (VRLA) and have a safety valve designed to release dangerous pressure. You can't and shouldn't open them to top up.
There are three forms of electrolyte: liquid dilute acid (which has been used for decades, and is known as "wet cell"), gel (which is basically liquid with a thickener to make it unspillable) and Advanced Glass Mat (AGM). This is more or less a paste version of the electrolyte, embedded in a glass-fibre mat. AGM and (particularly) gel usually need to be charged at lower speeds to prevent build-up of gas around the plates.
Normally, gel or AGM batteries are in VRLA cases, but not necessarily all of them are.
The advantage of gel is that you can turn the battery upside down with no risk of spillage. Ths might be an advantage for a caravan (where people carry batteries out to charge them), but I can't see it being useful for a motorhome. Because of selling in smaller numbers, gel batteries cost more. Some people seem to think that means they are better. That's not my opinion!
AGM batteries do cost more, and they do have advantages. When a battery is used in a stationary setting (such as a parked motorhome) it is possible for stratification to occur, so that different levels of the battery are at different states of charge. Apparently, having the electrolyte as a paste between the plates prevents this. The other advantage is that the glass fibre mat between the cells gives additional physical support to the plates. AGM batteries do cost more than wet cell. I'm not sure if the extra cost is worthwhile, but AGM VRLA batteries are the ones I have had best results from, and are what I would choose.
So once you have decided which sort of battery to buy, which one should you choose?
Firstly, remember that these are heavy things full of acid. They need to be very well secured inside the vehicle. In the event of an accident, you really don't want a plastic box fulled with acid, lead, explosive gases and a source of sparks flying around in the van. So you are limited to what will fit properly into a strong retaing clamp: probably the one the motorhome builder fitteded.
I've seen batteries held in place by 9mm of MDF board. I shudder to think of what might happen in an accident.
It is well worth fitting the battery with the biggest capacity there is space for. The bigger the battery, the less depleted your load will make it. And if you have two batteries, wire them in parallel to share the load, rather than keeping one as a spare.
Once you have decided the type and the size, how do you choose which make and model to buy? That's the easy bit! Buy the heaviest. The amount of lead is the important factor. Lead is by far the heaviest thing in the battery (unless it is filled with gold: if so, buy it anyway!)
Assuming a 12v 100Ah battery, a good one will weigh 30KG or more. An acceptable one will weigh about 25Kg. If it weighs 20KG or less, choose something else.
One caveat: we have two 12v 240Ah deep cycle batteries. They weigh in at 75KG each. This works out at over 31 KG per 100Ah, which is good, but frankly, 75KG is very heavy and difficult to deal with. Two batteries of half the weight would have been far easier (but these were an unmissable bargain). Heavier is good for a battery, but not for payload or handling.