Battery Wiring/Size

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Rngr275
Rngr275 Solar Expert Posts: 127 ✭✭
As some know I/we will be moving into our off grid Amish house over the next month. We will end up with either an off-grid PV battery system or the grid. It is truly unknown at this point but will be decided soon.

Anyway, until we get some sort of power we will be using an EccoTemp tankless water heater to take showers with. The water will come from our cistern by way of a Shurflo 12vDC pump which will feed a good size pressure tank. I will be using 2 6V golf cart batteries from Sam's club to power the pump. The literature for the pump says it can draw up to 5 amps and they recommend a 10amp fuse also. It has 16ga leads attached to the pump.

What gauge wire should I use for the battery to battery series connection? Can I splice on additional 16ga wire to the pump leads for the pump to battery connection? Are they talking a 10amp fuse like you would use in a car or something different?

An suggestions or diagrams will be greatly appreciated... I just want to do this right the first time.

Edit to add... is it worth buying a battery monitor or something similar?

McD

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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Battery Wiring/Size

    If this is the only load going on the batteries the wire between the two doesn't have to be any heavier than the wire to the pump.
    You might find it easier to buy a pre-made cable with a couple of lug ends on it and that will likely be 4 AWG or so.
    5 Amps is not a lot of current. 16 AWG is capable of handling it easily. The 10 Amp fuse can be an automotive type blade fuse, as it would meet the specs for both Voltage and current.
    One suggestion: add an on/off switch in the mix so that you can be sure the pump is off when working on it without having to take the wiring apart. The built-in pressure switch will try to turn the pump on any time pressure is below the "on" point, like when you're working on the plumbing.
  • ywhic
    ywhic Solar Expert Posts: 621 ✭✭
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    Re: Battery Wiring/Size

    Doing the same Eccotemp setup with the same 2088-XX pump myself soon..

    You should try and pay the extra bit and get the Eccotemp FLI-12-LP model for like $280..

    Its for indoors, had venting for air in/out, and has a 110v plug to just plug in and no batteries to mess with..

    You'll need to find a 1/2" NPT to whatever regulator (eccotemp sells the adaptor and kit for the L10 and L5 its all the same 1/2" NPT FWIW)or hard black pipe it and put your LP tank outside..

    If and when you get it figured out could you put up some pics and the parts for the water heater fittings would be a help to me..

    I'll be doing a switch also on my pump and the water heater (not that I don't trust it).. LoL..

    I figure turn the pump on before I draw my water from the above ground tank I will have.. (no well here).. as needed and save juice..

    They make a 110v 0.9 amp max rated 2088 pump also.. in case you want to go 110v with it.. (options..)
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
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    Re: Battery Wiring/Size

    It also matters how far the pump is from the battery bank... Typically, you would want to have less than 1.0 volt (Maximum--probably 0.5 volt maximum would be better) for a 12 volt system. Battery runs down to ~11.5 volts (under load) and the 1 volt wiring drop brings the pump voltage down to 10.5 (typically the bottom limit for most 12 volt devices).

    Also, you will want to look at how you are charging the battery bank... Ideally, my recommendation would be to have a 0.05 to 0.10 volt maximum drop in the wiring from the battery charger to the battery bank--So, you may end up with heavier gauge wiring at the battery bank/battery bus to keep the charger's voltage reading relatively accurate (you don't want much more than 0.2 volt drop or the battery bank will tend to charge much slower than the charger would be capable of).

    There are a few solar charge controllers with Remote Battery Voltage Sense Leads (Morning Star TS family, and Rogue 30 amp 12/24 MPPT controller are several that I am aware of)--These thin wires connect from the battery charger's voltage sense directly to the battery terminals (or battery bus) and allow the charge controller to accurately read the battery bank voltage without the issue of voltage drop in the "high" current leads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Rngr275
    Rngr275 Solar Expert Posts: 127 ✭✭
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    Re: Battery Wiring/Size

    This will be a temporary set up (1-2 months). I thought that I would have the batteries on a low movable cart incase I want/need to use them else where. SO it should be possible to keep the batteries and the charger fairly close together (less than 6ft) is my intention. I will be charging the batteries via my generator and a charger which will either be my current one (http://www.walmart.com/ip/Schumacher-SpeedCharge-15-Amp-Marine-Battery-Charger/13005745) or a new Iota.

    BB… Would it be better to just go heavier gauge with the wire just to be safe? Since the leads won’t be terribly long it is not a big cost difference. If so, what gauge and does it matter if it is stranded or not.

    Is buying the new charger worth it? 100 and some bucks is expensive but if it will be better for the batteries it might be worth it.

    How do you size the charger? 15amp, 30 amp… will one charge faster than the other?

    Is there a battery monitor that would be good for this application or should I just use a volt meter and be diligent about check the batteries?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Battery Wiring/Size

    Don't invest any more money than you have to before you determine if you really will be going off grid. That includes battery monitors, battery chargers, and heavier wires. If you do go with the utility power you're going to hate having several hundred dollars worth of stuff sitting around that you have no use for. This is a temporary solution; don't invest heavily in it.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
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    Re: Battery Wiring/Size
    Rngr275 wrote: »
    This will be a temporary set up (1-2 months). I thought that I would have the batteries on a low movable cart incase I want/need to use them else where. SO it should be possible to keep the batteries and the charger fairly close together (less than 6ft) is my intention. I will be charging the batteries via my generator and a charger which will either be my current one (http://www.walmart.com/ip/Schumacher-SpeedCharge-15-Amp-Marine-Battery-Charger/13005745) or a new Iota.

    Working from some basic rules of thumbs... Assuming a pair of 6 volt 220 AH golf cart batteries. The rate of charge should be around:
    • 220 AH * 0.05 rate of charge = 11 amps minimum rate of charge
    • 220 AH * 0.10 rate of charge = 22 amps "healthy nominal" rate of charge
    • 220 AH * 0.13 rate of charge = 28.6 amps maximum "cost effective" rate of charge (maximum long term/high current rate of charge typically)
    • 220 AH * 0.25 rate of charge = 55 amps not to exceed

    So, a 15 amp charger would be OK--but if you are running a genset, it will take longer to recharge vs a 22 amp charger.

    Also, you need to size the genset to the size of the load/charger--For example, with a typical AC charger:
    • 22 amps * 14.5 volts charging * 1/0.80 charger eff * 1/0.67 charger power factor = 595 VA rated genset minimum

    So, if you were running a Hond eu1000i (900 Watt/VA continuous rating), it would be a good fit. If you were running a 3.5 kW genset and no other loads, the 3.5 kW genset would be wasting quite a bit of fuel--especially as the charge controller tapered off.

    For the best use of generator resources, an efficient power supply with Power Factor ~ 1.0 would be a better choice... Here is a long thread about matching an eu2000i to the battery/charger capacities.
    BB… Would it be better to just go heavier gauge with the wire just to be safe? Since the leads won’t be terribly long it is not a big cost difference. If so, what gauge and does it matter if it is stranded or not.

    You can look at what is the "optimum" wire to use on the battery/bus interconnects... For a typical flooded cell battery, the maximum recommended surge current would be ~C/2.5, or for your bank:
    • 220 AH * 1/2.5 * 1.25 NEC safety factor = 110 Amps (minimum rated wiring/fusing to support full battery current)

    So, you can justify fairly heavy cable if you plan on using the maximum battery bank capacity now or in the future (plus keeping voltage drop low, especially on 12 volt systems, is always key to a happy user).

    The maximum rated (recommended) inverter would be around:
    • 220 AH * 12 volts * 1/8 max continuous battery load * 0.80 inverter eff = 264 watts maximum continuous rated load on inverter
    • 220 AH * 12 volts * 1/2.5 max surge * 0.80 inverter eff = 845 watts maximum surge for AC inverter output

    Since most good quality inverters support about 2x rated load for surge power, then you are looking at ~265 watt to 425 watt rated AC inverters to optimally use your battery bank's maximum power output.
    Is buying the new charger worth it? 100 and some bucks is expensive but if it will be better for the batteries it might be worth it.
    How do you size the charger? 15amp, 30 amp… will one charge faster than the other?

    A battery bank will take upwards of C/8 (recommended maximum of 12.5% or ~13% rated charging current) to C/2.5 during charging until the bank reaches ~80-90% of capacity. At that point the battery bank will be at the "absorb" voltage (~14.5 volts or so), and will be limiting charging current anyway.

    So, even if you have a vary "large" battery charger", will be be spending significant time (1-4 hours) at less than rated current capacity.

    If you where on a genset, I would be suggesting around a 20-30 amp battery charger for your use... Large enough to charge the battery bank fairly quickly--Not too large that you need to worry about overheating the battery if you deeply discharge and recharge (some people do recommend a very large battery charger--such as a ~55 amp--But I worry about battery bank getting hot, and you need a large AC genset to initially drive the AC charger--but much of the time the charger will be well below that maximum rated output--further wasting generator fuel--large generator driving small load).
    Is there a battery monitor that would be good for this application or should I just use a volt meter and be diligent about check the batteries?

    I like battery monitors--But they are fairly pricey, especially for a smaller battery bank and if this is only a temporary setup. $152 (plus shipping/insurance) for a less expensive Trimetric system may be hard to justify.

    If you are diligent and use good quality hydrometer to monitor your battery bank state of charge (ensure you get battery bank back over ~90% of charge a couple times a week), it will probably work OK for you.

    If you have highly variable loads and want monitor the battery bank so you only charge it when needed--A battery monitor is pretty nice.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset