Ok, first of all thanks for the ultra-fast response! what i meant was keep the 4 batteries together as 500amps You could connect them as 5 pairs of 24 volt units… the best way can only be determined by working out the volt drop for the maximum current drawn with the selected cable size and length. Is that acceptable? For example, if you have two 12V 100ah deep cycle RV batteries and you wire them together in parallel you end up with 12 volts and 200ah. I run the engine for about 15- 20 mins then switch off the engine, & indicator shows 12.6v. Getting it wrong will seriously hurt you. Hi John Wiring a battery in series is a way to increase the voltage of a battery. The capacity of the ampere per hour and the current capacity stays the same, but the voltage increases. like this one, the goal is to always have power to start the bike and when parked use the power of the extra battery or batteries to run the stereo for as long as I want, and when is time to go the bike will always start because I have not touched the power of the main battery….hope that makes sense. That said you really need a minimum of 200W of solar just to keep up with the fridge’s requirements over a 24 hour period. But….. the issue is with the compressor fridge. Am I overthinking this, or is there a blindingly obvious way to do it? Hi Allan Clear and concise. If the input rating on the inverter – no matter what the output wattage says 12 volts, you must only connect two batteries in parallel. Is it possible using your second diagram above to still hook up a dual battery setup provided that it’s same AGM gel battery of the same make-amperage- date of MFG, like you discussed in your article??? They were fitted new 2 years ago and wired incorrectly. When the solar panel starts to charge there is sometimes a clicking noise from the fuse box usually accompanied by the 240v alarm. I would have connected them the wrong way. Hi Jeff The parallel connection of two identical batteries allows to get twice the capacity of the individual batteries, keeping the same rated voltage. I need to be able to charge and power an ecg, BP and cardiac monitor as well as mobile phones, notebook computer. Each battery because of its differing age will have a differing charge profile and trying to charge them in a bank will invariably mean one battery is not charged to the maximum it can hold. is it ok to run a 12v tv connected to a separate leisure battery via 12v cable with crocodile clips going to a female cigarette socket connecter thanks? You will never charge the larger capacity battery at the optimal current as the smaller capacity battery will affect the output voltage of the smart charger. If you somehow flattened the batteries that are install in parallel (in the correct way). Outstanding !Thank you my friend. You are using an out of date browser. Which makes it a bit easier cabling. Also does this mean I would need different power level of battery’s or can they just be joined in parallel as you’ve described previously. You could add the larger battery to the two existing ones and run OK like that without any issues. However briefly, if both batteries are discharged to say 50%, on recharging the smaller capacity battery will reach a fully charged state first and the charged voltage of the smaller battery will shut off most smart chargers… the larger capacity battery will then never reach its full capacity and over a period of time will start to behave as the smaller capacity battery.. shortening it’s life in the process. Since this article was published I have received a lot of questions about connecting batteries. I have a Sears deep-cycle lead-acid 12v battery already which I've only used once or twice a year for certain power applications. For the devices… I’d use one 6mm cable to a fuse distribution strip, then one cable from each fuse to each separate device. Reply This is how I have planned to upgrade my system. I opt for a 24 volt leisure battery bank so that I could use a 24 volt inverter and for 12 volts… use a 24v to 12v converter. You will of course loose some life expectancy of the smaller batteries over a period of time and recharging cycles. For your application it has to work 100% every time. Tank you. They are designed for marine use and usually for banks of 800Ah upwards. Or would there be a better option? Hi Lee Hi Rob Now over a period of time, Battery A will always be ‘used’ slightly more than Battery B so at some point Battery A will have cycled more and be ‘aging’ faster than Battery B and it will eventually lead to problems and will need replacing. I would be working with 25mm trirated or welding cable as a minimum. i have a 100ah battery in my caravan and want to add another 100ah battery,if I connect them the way you have shown would it be ok. going to use portable sola panels. Hi Jason Ideally the two batteries need to be the same Ah rating otherwise they will discharge at different rates… and recharge at different rates. When you connect two batteries in parallel, you are able to double the capacity (amperage rating) while maintaining the same voltage. Thanks, Hi, I didn’t understand the bit about routing both cables through the same hole in the bodywork to stop the chance of eddy currents. Selecting a cable that is too small for the task and you might risk melting the cable insulation or damaging equipment due to voltage drop. “Suitcase” style inverter generators are relatively quiet and most can be converted to run on LPG if you don’t want to carry petrol. If the cables are rated for 100Amp…. S. I have a 2 year old motorhome that has 2 leisure batteries in parallel. Just double check the cable size… it might be 6mm2 with thin insulation. Hi John Why same batteries I’ve seen people who By connecting multiple deep cycle batteries, you can increase either the storage capacity or the voltage of any battery system. This is a handy site with a lot of info… http://www.selectsolar.co.uk/cat/165/choosing-a-solar-panel, Hi (Remember you know have daylight hours to collect via solar what the fridge takes in 24 hours). Morning Simon 2 x 120ah batteries at 12 volts gives 240ah…. As I said earlier, always use identical batteries – same make, same Ah rating and same manufacture date if possible. https://www.splitcharge.co.uk/product/redarc-bcdc1240d-battery-battery-charger-with-mppt-40a/, A Multicell Leisure Battery SFL-110 (Lead acid Sealed) http://www.multicell.co.uk/sfl-batteries.html, For habitaion heating/hot water I have a Webasto Thermo Top Evo Diesel Heating Kit Wiring Batteries in Parallel. The only other thing it could be is that one or both of your batteries are ‘aged’ and not keeping their charge or fully recharging. Hi Andrew it’s probably easier to work it out, so here goes… (if I can remember my college physics lessons!). Thanks I intend these to be connected to the solar for charging, but to be used completely independently from the original single battery with an output socket for USB and cigarette lighter adaptor for tv etc (as shown at website link below). The van has a 7 stage smart charger 240v and solar, will this be an OK set up for both batteries or will I keep them separate? If you have a two bank charger, personally I’d disconnect the batteries from the bank and charge then individually. Harry – Industrial Electrician – New Zealand. CSA = 25mm2 – 170 Amps (Outside diameter 9.8mm) Have a read through “Relay, VSR, SCR… what’s the difference?” to see if there is a better option that would suit your needs. Hi Is this a flawed system ie should I have solar disconnected whilst travelling or leave solar and disconnect vehicle alt.? Hence, you can connect two 12 volt batteries with 6Ah each, but you will still have a total of 24volts and 6Ah. Use this calculator to find out. This is for use in a tournament fishing boat that requires both batteries to be charged in short order – UPDATE – More info can be found on my blog here…. Because of space restrictions the two batteries are located 3.5 metres apart. Can I have two 200 amp alternators charging the batteries on my land rover for winch operation. The method I outlined above is suitable for most boat, motorhome or caravan situations. ( Log Out /  that is the MAXIMUM size fuse you should use. It means all the heavy duty leads can be made up with the same terminal’s crimped on. Remember when on electrical shore power the charger also acts as a regulated supply for any load, If you have the load on one side and the charing point on the other, that means the regulated power supply/charger will be supplying power through the batteries to the load and you will not get the benefits of a step charge that most modern caravan and motorhome units are capable off. As long as both batteries are charged equally using them together to start the engine will reduce the load on each battery and prolong the the life. Av ad a 110 to 120 no problems There is the option of using one battery as the main battery, then switching over to the second battery when required. Very useful information helped me understand how to wire my battery’s correctly thank you, Hi, I have a dual battery system in my vehicle with a deep cycle battery and solar panel on a camper trailer . https://www.dometic.com/en-gb/uk/products/food-and-beverage/fridges/motorhome-fridges/dometic-coolmatic-crx-65-s-_-79712. I could upgrade these to 35mm if necessary and run a neutral from aux battery 2 back to the neutral of the starter battery or the grounding post next to aux battery no1. I was thinking of purchasing an … So here is a quick “How To” guide with some explanations on the right way and the wrong way. At the source, or battery, you will read 12 volts using a voltage meter, but at the other end of the cable your voltmeter will only read 11.76 volts (12 volts -2%). i also know that fast charging affects battery life too but in order to get the same charge time i either need 36 amps charge current from my inverter which is not possible since its fixed either at 12 or 16 amp or my charge time of all 3 batteries has reduced by 1/3 compared to same inverter charging a single battery. I’d look for a suitable sized one that has a reasonable power density (how many W per square metre) and from a reputable make. I am also a bit confused about how heavy the connections cable should be. This would also mitigate having to run multiple earths from various points back to the second battery (this is sometimes how they do it with house batteries on boats when they are located in obscure sections of the hull). I believe I must connect them in parallel. Moving either the earth or +ve to the opposite end of the battery bank means the differential voltage drop is eliminated and the voltage drop is the same for both (all) batteries. You can use batteries of different ages and Ah ratings but this will inevitably shorten the life of both batteries. Please Give me Reply. What you might have seen is the +VE of one battery connected to the -VE of another… this is known as a series connection and is used to connect two batteries to increase the voltage. I’m a newbie when it comes to electrics, so would really appreciate some advice please.. So many variables here. hello simon We will still have the voltage drop – although it will be smaller as the charging current is a lot less than the cranking current for starting your engine, lets call it 1/4 volt (0.25 volts) on each cable, so now the engine is running and the batteries are charging .. but Battery A gets the full charging voltage – 14.0 volts for example but Battery B only gets 13.0 volts, so not really enough. If you have look on Part 2… https://caravanchronicles.com/guides/how-to-connect-two-batteries-in-parallel-part-2/ you can see a drawing for connecting three batteries. If you take the feed and return from just one battery you get differential voltage drops between the batteries on both charge and discharge. Do you think they may be connected? So what you are suggesting assuming the inverter is 80% efficient and the motor/alternator is 80% efficient means to run your heaters for one hour (over the course of an evening) off the battery pack, to put back that energy using the motor you have would require running your motor/generator for over 4 hours each day. I am a complete newbie regarding campervan electrics(or any other kind of electrics for that matter) For example if you connect two of our 12 Volt, 10 Ah batteries in series you will create one battery that has 24 Volts and 10 Amp-hours. the fact is once they are in parallel it doesn’t matter which way you connect the battery because there is no battery A and no battery B they become one battery. What size cable should I use to bridge them together? Additional fusing might be needed to protect the wiring downstream. Hi Ian In a Parallel Configuration the batteries are wired per the diagram below and the result would be a doubling of the capacity while the voltage remains the same. The difference here is that this car runs a dynamo, not an alternator. It would be non standard and something I would not recommend. The existing cable looks about 4mm2 – would similar cable be sufficient to connect the batteries or does it have to be heavier? If you drain 10% capacity out the battery bank, the lower capacity battery will have an individual drain different to the larger capacity battery. Hi Norrie I have attached a PDF drawing (Below) of the above drawing that you can down load and print out with the wiring diagrams above. Well the most obvious is to simply connect a new battery to the original battery using some cable and new battery terminal clamps like this…. Have you tried an online cable sizing calculator ? However, if you are installing an inverter, you need to size the cables between the two batteries and between the batteries and inverter to take the maximum current that the inverter is capable of drawing. so can 800 watt inverter hold two batteries with parallel combination instead of series……???????? I’m planning to keep the ‘home’ part of the motorhome as mainly 12 volts as there are plenty of 12v appliances available. However after lots of head scratching he came up with this: where: Here is a link to an example: https://www.limekilnchandlers.co.uk/index.php?_a=product&product_id=65. Thanks in advance – and thanks for an excellent blog Hi Doug Depending on the size (wattage) of the solar panel though you might need a separate regulator. Cheers mark. You can get battery post terminals that are quick disconnect. Does battery need a metal base. Wiring a battery in series is a way to increase the voltage of a battery. but 2 x 12 volts in series sill only gives you 120ah but you draw less current for a given load, so it all balances out. Then I’ll be turning my attention to leisure batteries, so that over the winter months I’ll be able to run the 12v diesel heater that I’m going to install and work in there without suffering from hypothermia. Pairing two older batteries is always better than pairing one old one to a brand new one as the life of the new one always seems to be reduced almost to the life left in the old one. curiosity always gets the better of me. If one battery develops a problem, a very large current can pass from the good battery to the dud, resulting in an explosion. Care should be taken when paralleling batteries. Many thanks again for the advice, and I will look up that dynamo regulator too. remember that with DC, you want the total length of +ve & -ve added together. Hopefully there is suitable fusing close to each battery! It’s worth performing this simple test first. With the same battery I can Take + and – of one single battery means the Voltage will be 12 V but How much will be the Current. Is this correct please? If I had that set up, I might lean towards how many marine setups are done and have two separate banks and a changeover switch between the two… there are battery master switches that have “Off – Bank 1 – Bank 2 – Both” options. Its me again with more questions. I know not ideal but I couldn’t afford the Trojan batteries i’d of preferred :). Ideal for 12V and 24V systems such as campers, van conversions, and solar projects. Great help and advice thanks. Looking at the advice given, it would seem that I should charge both fully first. Very many thanks for the article and ALL the info, it really does make for great reading. You also want to use these four batteries to power a 250 watt 12 volt motor that is coupled to an alternator and use that to recharge the four batteries. The nominal capacity for automotive use cable is:-, 4mm2 cable is rated at 27 Amps – so fuse at 20 Amps Tim. I do have 1 question regarding power inverters. A bank charger will charge each individual battery. You can see that running parallel at 12 volts for a 120 watt load gives a result of 20 but if you run in series with a 240 watt load you get a result of 10. If you wire two 200ah 6 volt batteries together in series you end up with 12 volts but the amp hours remain 200. I is the discharge current Only if you connect two 12 volt 90Ah batteries in parallel do you get 12 volt 180Ah capacity. The other side of this is charging. Hi, didn’t know about 2 different ah batteries in parallel. LED lighting. My questions are, first, have i got the right leisure battery, second would i benefit from having a second battery installed( i have space right next to the existing). So simply by moving one connection we have evened out the volt drop to both batteries. The best way of connecting three batteries to minimise loss is shown in part two https://caravanchronicles.com/guides/how-to-connect-two-batteries-in-parallel-part-2/. Now after reading your info will stop for a moment and work out a new plan & cost. Hi The only thing that crossed my mind was they might have had past experience where the grounding of second batteries was not up to standard. I’m not too familiar with the AU wire sizes but I think 8B&S rating is 72 Amps, so as long as you don’t intend pulling more that 70 Amps continuous from you battery bank is should be OK. Hi im glad I found this as I thought this was how it should be done. The only way to connect the two batteries for your application is in parallel. As a matter of interest there have been a number of incidents that upon reflection might be attributable to the wiring error. Are there any negative effects to using both new batteries simultaneously to start my engine. All installations are different. I have a 2018 VW T6 transporter (campervan converted).it has a Euro 6 engine with stop start. Thanks Ash. I have two matched 180ah batteries with a pwm controller(60amp) and two 250w solar panels wired in parallel? Just put 2nd battery in my caravan as you have shown on your direction ,i have 140 kw ,solar panel is that sufficient, if it is when i had 1 battery we use to run out of power free camping will the 2nd battery give me additional length of power if so approximately percentage wise how much,i have 2 100amp batteries. During shading/night (when there is no generating power from solar panels) the battery stored energy … Discharge/recharge is not proportional to individual battery capacity in a bank as effectively you don’t have individual batteries. Or am I overlooking something? Hi Dino Something else to consider is a solar panel array to recharge your battery bank, but to put back the amount of energy you take out running the heaters for one hour over the course of a night will require a big setup to recover that amount of energy in daylight hours. I can’t think of any reason why it would say that. I'd use cube fuses on the battery terminals... Other than the maximum load current to be drawn, the most important criterion for battery cable size is usually what kind of charger is being connected and where, because voltage drop during charging is extremely detrimental to charging performance and could result in one battery receiving less charge than the other. Hello again Simon, I have a question about connection the wiring to load, how about connecting it to the opposite terminals (ones that are not connected to the power source) wouldn’t it be more efficient as they wont be connected to the same terminals where power is coming in? Could you please give me some ides of what you think is wrong I have a 800 VA inverter and this time i connected three 75 AMPs batteries in parallel. It is also worth checking that all the cable terminations are in good condition and the cable used is of the minimum size required. In relation to the batteries; what cable would you recommend? Would fuses be a useful precaution and if so how large would they need to be to connect 2 X 110Ah batteries running lights and a TV in a motor home? Our vehicles alt charges the the dual battery from which a cable runs to Anderson plug connection to our camper battery while it is still connected to the solar panel . It much better to use a proper bit of wire as the earth return path I think. Hi Lee Hi For the connecting wires between the two batteries I used 8B&S cable. From the starting battery I have a 35mm cable to (inline isolator first) the aux battery no1. Feels also a bit safer to put a 30A fuse on each battery. I have 2x 12volt 100ah batteries @20hour rating and connected in parallel powering 120 watts of lighting. So if you wanted to run a 150 watt heater at 12 volts that’s 12.5 Amps, but to run a 150 watt heater at 220 volts through an inverter from the same 12 volt source you would be drawing around 15.5 amps from the 12 volt supply. Also something to consider now is that with modern vehicle electronics there are special jump leads available that protect sensitive electronics from potential damage from jump starting. This is sometimes used if the cable isn’t running through lots of bodywork or is inside flexible conduit. I would recommend fuses are installed at the +ve end of each cable. I want to connect two batteries together to sit on their own (not wired into the car’s systems at all) with an ARB fridge ‘plugged in’ to them. 147 Amp hours * 4 = 588 Amp hours. I have a friend that runs a classic MG that I gave exactly the same advice to and he’s been happy for the last few years with the setup. The following wiring configuration requires four 6V batteries. four diodes in total. I have just setup two agm batteries in my van on a solar system and did not know there was a right and wrong way to connect two batteries in parallel. Connecting batteries of different voltages in parallel… A very useful article, thank you. Tilting panels make a huge difference. I have a 130ah and a 105ah deep cycle agm, same brand in my camper trailer. Thank you for an unexpected quick reply, you obviously treat Sunday like I do never switch off. Change ). So, you would only need one gauge, and the gauge would need to be the system voltage of the battery pack. Hi Simon, I also have an inverter which I have also wired to the same two terminals ( Neg Batt 1, Poss Batt 2) is this the best wiring? Whenever I turn on our TV it will run for about 15 mins and then the power will go off and the alarm on the inverter goes indicating low battery power. Do I need to run an earth lead to the chassis for both batteries? Hi If you don’t need pure starting power (cranking amps) from your battery, there may be other options available that would give you split charging and the ability to combine them if required. You can usually get them checked at a local battery supply outlet. Hi, I found this info very useful. Thanks for that, do you need an earth to the chassis of the trailer to run my accessories?