Batteries for Overland/camping vehicle Aux power  - Page 1
Forum section: electronics / projects

author: wblackledg
author_rank: Regular Contributor
timestamp: July 29, 2018, 02:15:15 pm
content: Sorry if this isn't the right forum, but I wanted to bounce this idea off of people who know a lot more about electrical/electronics than most people.I want to install an Auxiliary battery in my Jeep for various needs while camping (CPAP, etc). I had only considered a deep cycle Marine type battery as the Aux battery until I watched this video with Andrew St Pierre White.Go to about 8:00 into the video. He recommends for short trips (~1 hour) a high cycle battery would be better than a deep cycle battery.Shorter trips (not all day drives), would fit closer to what I'm planning.Question(s)1. Does what he suggest make sense?2. Would a hybrid system, of 1 Deep Cycle, and 1 High Cycle as an Aux system (excluding the starting battery) make more sense, or would having a deep cycle wired in parallel along with the high cycle battery defeat the purpose of quick charging?I was planning on using a battery isolator like this one.https://www.amazon.com/Smart-Battery-Isolator-Voltage-Sensitive/dp/B00400IYTKthanks in advance!

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2018, 05:00:19 pm
content: I never heard that it was a big problem but there is at least some truth to it.Deep cycle batteries have a higher internal impedance than starter batteries. Usually this does not matter because they will be used either in applications where charge and discharge currents are lower and times are higher or they are used in larger banks to support overnight loads.Faster deep cycle charging does rely on a higher charging voltage which is inconvenient when the power source is a vehicle charging system intended to work with starter batteries.I thought his argument about the economics was more interesting. Deep cycle batteries do tend to cost a lot more making starter batteries more economical in light duty applications. Personally in a remote application, I would want duplicate batteries anyway so that in the event of starter battery failure, the batteries can be swapped.

author: wblackledg
author_rank: Regular Contributor
timestamp: July 29, 2018, 05:56:34 pm
content: Quote from: David Hess on July 29, 2018, 05:00:19 pmthanks for the response. I believe those battery isolation units I linked to have an override option to recharge the starting battery from the deep cycle/Aux battery when and if Needed.thanks

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2018, 07:00:35 pm
content: Quote from: wblackledg on July 29, 2018, 05:56:34 pmCharging from one battery to another is pretty slow because of the low voltage. Usually the idea is to just charge the starter battery enough that it can start the vehicle and then the vehicle charges it quickly.

author: ConKbot
author_rank: Super Contributor
timestamp: July 30, 2018, 01:42:50 pm
content: Some truth, some misunderstanding there. Different "12V" lead acid batteries have different voltages. Pop a 12V AGM deep cycle cell (no-spill, no maintenance, wonderful! ) in a camper with just a charging/isolation relay with a flooded main battery, and it won't charge fully, and probably die sooner as a result.https://www.energymatters.com.au/components/battery-voltage-discharge/If a flooded deep cycle has the same charge voltages as the engine battery, it will be fine.Also, on top of matching battery voltages, an alternator that does 3-stage charging is useful for charging up batteries. A float of 14.5V is good for quickly refilling a starting battery when a car does mostly short trips, if you're driving for hours, the battery is sitting there gassing/cooking the entire time. I had a 2008 Chevy with an alternator that charged up to 14.5v the it eased off to 13.8v after the charge was completed. The battery lasted for over 9 years, and the car was sold before it needed replacing.Charging a lead acid battery is dead simple (transformer with a "soft" output and rectifier) Charging a lead acid battery well is hard. Cyclic and float voltages varying with chemistry(pure lead, lead/antimony, lead/calcium plates, etc), temperature voltage compensation, occasionally gassing flooded ones for balancing.

author: splin
author_rank: Frequent Contributor
timestamp: July 31, 2018, 10:45:14 pm
content: Quote from: wblackledg on July 29, 2018, 02:15:15 pmFrom the Rolls battery user manual for flooded cells:QuoteThis is for the bulk recharging phase (between 0% to 80% state of charge), the last 20%, absorbtion phase, taking much longer as the current drops off to almost zero.The problem here is that it will take between 3 and 6 hours bulk charging from 20% SOC to 80% and absorbtion will take between 2.1 and 4.2 hours, meaning between 5 and 10 hours to fully charge an 80% discharged battery. If you have only a 1 hour drive then you can only put in between 10% and 20% charge depending on how full the battery is at the start.If you are only going to be able to use 10 to 20% of a battery's capacity then it doesn't need to be deep cycle. On the other hand, starter batteries have fairly short cycle lifes at even moderate depth cycles and their life is seriously compromised by even the occasional deep discharge (especially AGM and Gel batteries).This is made worse by the charging voltages required. The Rolls manual states 15V at 25C for bulk and absorbtion stages, but many vehicles, especially older ones, may have a charging voltage of 14.4 to 14.7V as a compromise between recharging the battery quickly but not over-cooking them when the battery is fully charged. The lower charge voltage from the alternator, coupled with the voltage drop over the relatively long wiring run to the towed vehicle's battery (and through the towbar plug/socket) mean the charge rate may be very low.This may be less of a problem on modern vehicles designed for Ca-Ca batteries - as are most(?) Fords - which use a higher charge voltage of 15V+.Many recent European vehicles have an additional problem (which probably won't be an issue for the OP) in that they are fitted with start-stop systems to improve fuel efficiency in town driving. These systems don't charge the starter battery more than about 75% to allow regenerative braking to dump some of the braking energy into the battery. Obviously it can't do this if the battery is fully charged. This means that the vehicle BMS will reduce the charging voltage to perhaps 13.5V when the battery gets to 75% charged. The auxiliary battery may then get almost no charge at all at that point and almost certainly won't get above 75% at best.A way around this is to use a boost convertor set to say 15V at the towed vehicle's battery. That could be a cheap Ebay module with CV and CC to limit the peak current. Two or more could be paralleled to increase the current output as required as the current limits will cause the current to share between them. This isn't ideal as the charge voltage ideally should be temperature compensated. It wouldn't reduce to the float voltage either but that won't be an issue if the driving time is so short that the battery rarely gets fully charged.You can get systems such as Sterling Power's Wildside BBC1225 which provide voltage boosting and proper battery charging regimes but they aren't cheap.Another solution is to use an auxiliary battery that can take a higher charge rate. AGM batteries can have charge rates up to .35 x C and Lithium-ion batteries can charge at between .5 and 1C - but then you have other issues to deal with including charge voltages which may be incompatible with the starter battery.Lithium batteries are expensive but don't suffer from long periods at low state of charge like lead batteries which need to be recharged as soon as possible. They also have the possibility to swap with, (or use to jumper) the starter battery in an emergency.QuoteIt doesn't get round the problem that the deep cycle battery simply can't be recharged quickly enough.Solar panels may be a better alternative, in summer at least, as they can provide charge over sufficiently long periods to (near) fully recharge the battery during the long but low current absorbtion phase which would be expensive and wasteful to do with a generator, or much worse, an idling tow vehicle.