BATTERIES AGAIN

[dsw]

I have just bought 3 leisure batteries 110 Ah each and will be putting them in parallel to make 330 Ah total but what I need to know is will my 70Ah alternator charge the bank and keep it at a reasonable state ? I wont be actually using no where near 330 Ah but I want it there just in case I decide to do a nights fishing and leave the 600w inverter on to power 2 x 58w fluorescent lights, the engine batteries are totally separate so no worries about them as they are charged by the 24v alternator.

 

[alan]

I have read (somewhere) that the maximum battery capacity should be about three times the alternator output power in order to achieve charge in a reasonable time. There are some calcs in Febs PBO "Ask the Experts" on page 22, which seems to support this 3X.

Alan.

 

[pvb]

Yes, a 70A alternator will work fine with a 330Ah battery bank. Theoretical multiples of 3x or 4x don't really seem to work in the real world - you're unlikely to be able to push 70A into the batteries for long. If you feel it's not recharging effectively within your engine running time, you might consider using a smart regulator (Adverc, Sterling, etc) to boost its output voltage and hence the charging current.

 

[Bilgediver]

Check the voltage you get during charge and if it only hovers around 13.8 would suggest you at least get a regulator to give 14.4 to ensure good equalisation or get a smart charger.

 

[Trundlebug]

My gut feel, without doing all the maths and spouting equations at you is that the alternator itself should be fine and quite adequate, but it's worth bearing in mind a few key factors.
Firstly, as long as your engine is on a separate battery you're fine anyway - you should always be able to start up and charge.

Secondly, it's your charging system that will influence the alternator output more than the actual capacity of the alternator itself. Assuming you have a split charging system of some sort, you are very unlikely to be charging at 70Amps at any point even if the batteries are significantly discharged so I wouldn't worry. More important is the voltage the regulator in the alternator is set to. Most likely the normal sort of settled charging current will be around 20A (very roughly, lots of variables etc etc before all the other experts shout me down!) so nowhere near the full alternator capacity.

Finally, how long will you be running after discharging the batteries? You need to ensure that they will have sufficient time to recharge, (several hours required, e.g. 4-8 after a significant discharge) and that may influence the charging system required.

In summary though the alternator should be fine.

 

[JerryHawkins]

Think about how to connect them...

Before you connect them, there are several ways... have a look at this page and you'll see what I mean.

http://www.smartgauge.co.uk/batt_con.html

Jerry

 

[saltwater_gypsy]

Re: Think about how to connect them...

Thanks Jerry Hawkins for the link. Very interesting. It seems a simple problem until you include the internal resistance of the batteries and the wiring. I have got nine batteries so I'll lie awake tonight working out the best way to connect them!!!

 

[William_H]

Re: Think about how to connect them...

The adequacy of the charging system will depend on your power needs. If the battery bank is only lightly discharged as you suggest then it will be very adequate. However if you deeply discharge the batteries then obviously they will take a lot of engine running to recharge.
When you start recharging from a deep discharge the alternator will be asked to provide near maximum current which will cause the output voltage to fall so current into the batteries will fall. The actual output voltage will be able to rise to equal that of the batteries. Sort of self balancing. The alternator may get pretty warm but should not be damaged by overload. (because of this self limiting of current)

Once the batteries start to recover the current they will take will be less but because there are so many batteries the current will be proportionally higher for longer.
So by adding batteries you get something approaching the effect of a smart or stepped charging system. The end result is more current into the batteries for a given engine running time. Which results in less engine running to replace a given usage.

So yes you should be fine. Check the temperature of the body if you are really loading the alternator. You may end up with a shorter life of the alternator. And a bigger current alternator would be running at further below max current so much cooler and longer life. But if you hardly ever really discharge the batteries this should not be a problem. olewill

 

[charles_reed]

The only problem with the proposals on smartguage site

is that you can't do it that way if you have an alternative switch over from one battery to the next.